Internet DRAFT - draft-ietf-lsvr-bgp-spf
draft-ietf-lsvr-bgp-spf
Network Working Group K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Standards Track A. Lindem
Expires: 14 September 2023
S. Zandi
LinkedIn
W. Henderickx
Nokia
13 March 2023
BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-22
Abstract
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP as their single routing protocol for both their fabric routing
and their Data Center Interconnect (DCI) routing. This document
describes extensions to BGP to use BGP Link-State distribution and
the Shortest Path First (SPF) algorithm. In doing this, it allows
BGP to be efficiently used as both the underlay protocol and the
overlay protocol in MSDCs.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
Patel, et al. Expires 14 September 2023 [Page 1]
Internet-Draft BGP Link-State SPF Routing March 2023
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Base BGP Protocol Relationship . . . . . . . . . . . . . . . 6
3. BGP Link-State (BGP-LS) Relationship . . . . . . . . . . . . 7
4. BGP SPF Peering Models . . . . . . . . . . . . . . . . . . . 7
4.1. BGP Single-Hop Peering on Network Node Connections . . . 8
4.2. BGP Peering Between Directly-Connected Nodes . . . . . . 8
4.3. BGP Peering in Route-Reflector or Controller Topology . . 9
5. BGP Shortest Path Routing (SPF) Protocol Extensions . . . . . 9
5.1. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . 9
5.1.1. BGP-LS-SPF NLRI TLVs . . . . . . . . . . . . . . . . 10
5.1.2. BGP-LS Attribute . . . . . . . . . . . . . . . . . . 10
5.2. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . 10
5.2.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . 11
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Capability
TLV . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV . . 12
5.2.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . 13
5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length
TLVs . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV . . 14
5.2.3. IPv4/IPv6 Prefix NLRI Usage . . . . . . . . . . . . . 15
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status
TLV . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . 16
5.3. NEXT_HOP Attribute Manipulation . . . . . . . . . . . . . 17
6. Decision Process with SPF Algorithm . . . . . . . . . . . . . 18
6.1. BGP SPF NLRI Selection . . . . . . . . . . . . . . . . . 19
6.1.1. BGP Self-Originated NLRI . . . . . . . . . . . . . . 20
6.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 20
6.3. SPF Calculation based on BGP-LS-SPF NLRI . . . . . . . . 21
6.4. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 25
6.5. NLRI Advertisement . . . . . . . . . . . . . . . . . . . 25
6.5.1. Link/Prefix Failure Convergence . . . . . . . . . . . 25
Patel, et al. Expires 14 September 2023 [Page 2]
Internet-Draft BGP Link-State SPF Routing March 2023
6.5.2. Node Failure Convergence . . . . . . . . . . . . . . 26
7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 27
7.1. Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . . 27
7.2. Processing of BGP-LS-SPF NLRIs . . . . . . . . . . . . . 28
7.3. Processing of BGP-LS Attribute . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10. Management Considerations . . . . . . . . . . . . . . . . . . 32
10.1. Configuration . . . . . . . . . . . . . . . . . . . . . 32
10.2. SPF Algorithm Consistency . . . . . . . . . . . . . . . 32
10.3. Link Metric Configuration . . . . . . . . . . . . . . . 32
10.4. Adjacency End-of-RIB (EOR) Marker Requirement . . . . . 32
10.5. backoff-config . . . . . . . . . . . . . . . . . . . . . 33
10.6. Operational Data . . . . . . . . . . . . . . . . . . . . 33
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 33
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informational References . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP [RFC4271] as their single routing protocol for both their fabric
routing and their Data Center Interconnect (DCI) routing [RFC7938].
This document describes an alternative solution which leverages BGP-
LS [I-D.ietf-idr-rfc7752bis] and the Shortest Path First algorithm
used by Internal Gateway Protocols (IGPs).
This document leverages both the BGP protocol [RFC4271] and the BGP-
LS [I-D.ietf-idr-rfc7752bis] protocols. The relationship, as well as
the scope of changes are described respectively in Section 2 and
Section 3. The modifications to [RFC4271] for BGP SPF described
herein only apply to IPv4 and IPv6 as underlay unicast Subsequent
Address Families Identifiers (SAFIs). Operations for any other BGP
SAFIs are outside the scope of this document.
This solution avails the benefits of both BGP and SPF-based IGPs.
These include TCP based flow-control, no periodic link-state refresh,
and completely incremental NLRI advertisement. These advantages can
reduce the overhead in MSDCs where there is a high degree of Equal
Cost Multi-Path (ECMPs) and the topology is very stable.
Additionally, using an SPF-based computation can support fast
convergence and the computation of Loop-Free Alternatives (LFAs).
Patel, et al. Expires 14 September 2023 [Page 3]
Internet-Draft BGP Link-State SPF Routing March 2023
The SPF LFA extensions defined in [RFC5286] can be similarly applied
to BGP SPF calculations. However, the details are a matter of
implementation detail. Furthermore, a BGP-based solution lends
itself to multiple peering models including those incorporating
route-reflectors [RFC4456] or controllers.
1.1. Terminology
This specification reuses terms defined in section 1.1 of [RFC4271]
including BGP speaker, NLRI, and Route.
Additionally, this document introduces the following terms:
BGP SPF Routing Domain: A set of BGP routers that are under a single
administrative domain and exchange link-state information using
the BGP-LS-SPF SAFI and compute routes using BGP SPF as described
herein.
BGP-LS-SPF NLRI: This refers to BGP-LS Network Layer Reachability
Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI
(Section 5.1) and is being used for BGP SPF route computation.
Dijkstra Algorithm: An algorithm for computing the shortest path
from a given node in a graph to every other node in the graph.
1.2. BGP Shortest Path First (SPF) Motivation
Given that [RFC7938] already describes how BGP could be used as the
sole routing protocol in an MSDC, one might question the motivation
for defining an alternate BGP deployment model when a mature solution
exists. For both alternatives, BGP offers the operational benefits
of a single routing protocol as opposed to the combination of an IGP
for the underlay and BGP as an overlay. However, BGP SPF offers some
unique advantages above and beyond standard BGP distance-vector
routing. With BGP SPF, the standard hop-by-hop peering model is
relaxed.
A primary advantage is that all BGP SPF speakers in the BGP SPF
routing domain have a complete view of the topology. This allows
support for ECMP, IP fast-reroute (e.g., Loop-Free Alternatives),
Shared Risk Link Groups (SRLGs), and other routing enhancements
without advertisement of additional BGP paths [RFC7911] or other
extensions.
Patel, et al. Expires 14 September 2023 [Page 4]
Internet-Draft BGP Link-State SPF Routing March 2023
With the BGP SPF decision process as defined in Section 6, NLRI
changes can be disseminated throughout the BGP routing domain much
more rapidly. The added advantage of BGP using TCP for reliable
transport leverages TCP's inherent flow-control and guaranteed in-
order delivery.
Another primary advantage is a potential reduction in NLRI
advertisement. With standard BGP distance-vector routing, a single
link failure may impact 100s or 1000s prefixes and result in the
withdrawal or re-advertisement of the attendant NLRI. With BGP SPF,
only the BGP SPF speakers corresponding to the link NLRI need to
withdraw the corresponding BGP-LS-SPF Link NLRI. Additionally, the
changed NLRI is advertised immediately as opposed to normal BGP where
it is only advertised after the best route selection. These
advantages provide NLRI dissemination throughout the BGP SPF routing
domain with efficiencies similar to link-state protocols.
With controller and route-reflector peering models, BGP SPF
advertisement and distributed computation require a minimal number of
sessions and copies of the NLRI since only the latest version of the
NLRI from the originator is required. Given that verification of the
adjacencies is done outside of BGP (see Section 4), each BGP SPF
speaker only needs as many sessions and copies of the NLRI as
required for redundancy. Additionally, a controller could inject
topology that is learned outside the BGP SPF routing domain.
Given BGP-LS NLRI is already consumed [I-D.ietf-idr-rfc7752bis], this
functionality can be reused for BGP-LS-SPF NLRI.
Another advantage of BGP SPF is that both IPv6 and IPv4 can be
supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF NLRIs.
In many MSDC fabrics, the IPv4 and IPv6 topologies are congruent
(refer to Section 5.2.2 and Section 5.2.3). Although beyond the
scope of this document, multi-topology extensions could be used to
support separate IPv4, IPv6, unicast, and multicast topologies while
sharing the same NLRI.
Finally, the BGP SPF topology can be used as an underlay for other
BGP SAFIs (using the existing model) and realize all the above
advantages.
Patel, et al. Expires 14 September 2023 [Page 5]
Internet-Draft BGP Link-State SPF Routing March 2023
1.3. Document Overview
The document begins with sections defining the precise relationship
that BGP SPF has with both the base BGP protocol [RFC4271]
(Section 2) and the BGP Link-State (BGP-LS) extensions
[I-D.ietf-idr-rfc7752bis] (Section 3). The BGP peering models, as
well as the their respective trade-offs are then discussed in
Section 4. The remaining sections, which make up the bulk of the
document, define the protocol enhancements necessary to support BGP
SPF including BGP-LS Extensions (Section 5), replacement of the base
BGP decision process with the SPF computation (Section 6), and BGP
SPF error handling (Section 7).
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Base BGP Protocol Relationship
With the exception of the decision process, the BGP SPF extensions
leverage the BGP protocol [RFC4271] without change. This includes
the BGP protocol Finite State Machine, BGP messages and their
encodings, processing of BGP messages, BGP attributes and path
attributes, BGP NLRI encodings, and any error handling defined in the
[RFC4271] and [RFC7606].
Due to the changes to the decision process, there are mechanisms and
encodings that are no longer applicable. While not necessarily
required for computation, the ORIGIN, AS_PATH, MULTI_EXIT_DISC,
LOCAL_PREF, and NEXT_HOP path attributes are mandatory [RFC4271] and
are validated. Unless explicitly specified in the context of BGP
SPF, all other attributes SHOULD NOT be advertised. However, if they
are advertised, they will be accepted, validated, and propagated
consistent with the BGP protocol.
Section 9 of [RFC4271] defines the decision process that is used to
select routes for subsequent advertisement by applying the policies
in the local Policy Information Base (PIB) to the routes stored in
its Adj-RIBs-In. The output of the Decision Process is the set of
routes that are announced by a BGP speaker to its peers. These
selected routes are stored by a BGP speaker in the speaker's Adj-
RIBs-Out according to policy.
Patel, et al. Expires 14 September 2023 [Page 6]
Internet-Draft BGP Link-State SPF Routing March 2023
The BGP SPF extension fundamentally changes the decision process, as
described herein. Specifically:
1. BGP advertisements are readvertised to neighbors immediately
without waiting or dependence on the route computation as
specified in phase 3 of the base BGP decision process. Multiple
peering models are supported as specified in Section 4.
2. Determining the degree of preference for BGP routes for the SPF
calculation as described in phase 1 of the base BGP decision
process is replaced with the mechanisms in Section 6.1.
3. Phase 2 of the base BGP protocol decision process is replaced
with the Shortest Path First (SPF) algorithm, also known as the
Dijkstra algorithm.
3. BGP Link-State (BGP-LS) Relationship
[I-D.ietf-idr-rfc7752bis] describes a mechanism by which link-state
and TE information can be collected from networks and shared with
external entities using BGP. This is achieved by defining NLRI
advertised using the BGP-LS AFI. The BGP-LS extensions defined in
[I-D.ietf-idr-rfc7752bis] make use of the decision process defined in
[RFC4271]. Rather than reusing the BGP-LS SAFI, the BGP-LS-SPF SAFI
(Section 5.1) is introduced to insure backward compatibility for the
BGP-LS SAFI usage.
The BGP SPF extensions reuse the format of the Link-State NLRI, the
BGP-LS Attribute, and the TLVs defined in [I-D.ietf-idr-rfc7752bis].
The usage of is described in Section 5.2. The usage of other BGP-LS
TLVs or extensions is not precluded and is, in fact, expected.
However, the details are beyond the scope of this document and may be
specified in future documents.
The rules for setting the NLRI next-hop path attribute for the BGP-
LS-SPF SAFI follow the BGP-LS SAFI as specified in section 3.4 of
[I-D.ietf-idr-rfc7752bis].
4. BGP SPF Peering Models
Depending on the topology, scaling, capabilities of the BGP SPF
speakers, and redundancy requirements, various peering models are
supported. The only requirement is that all BGP SPF speakers in the
BGP SPF routing domain adhere to this specification.
Patel, et al. Expires 14 September 2023 [Page 7]
Internet-Draft BGP Link-State SPF Routing March 2023
4.1. BGP Single-Hop Peering on Network Node Connections
The simplest peering model is the one where EBGP single-hop sessions
are established over direct point-to-point links interconnecting the
nodes in the BGP SPF routing domain. Once the single-hop BGP session
has been established and the Multi-Protocol Extensions Capability
with the BGP-LS-SPF AFI/SAFI has been exchanged [RFC4760] for the
corresponding session, then the link is considered up from a BGP SPF
perspective and the corresponding BGP-LS-SPF Link NLRI is advertised.
An End-of-RIB (EoR) Marker [RFC4724] for the BGP-LS-SPF SAFI MAY be
expected prior to advertising the BGP-LS Link NLRI for to peer.
A failure to consistently configure the use of the EoR marker can
result in transient micro-loops and dropped traffic due to incomplete
forwarding state.
If the session goes down, the corresponding Link NLRI are withdrawn.
Topologically, this would be equivalent to the peering model in
[RFC7938] where there is a BGP session on every link in the data
center switch fabric. The content of the Link NLRI is described in
Section 5.2.2.
4.2. BGP Peering Between Directly-Connected Nodes
In this model, BGP SPF speakers peer with all directly-connected
nodes but the sessions may be between loopback addresses (i.e., two-
hop sessions) and the direct connection discovery and liveliness
detection for the interconnecting links are independent of the BGP
protocol. For example, liveliness detection could be done using the
BFD protocol [RFC5880]. Precisely how discovery and liveliness
detection is accomplished is outside the scope of this document.
Consequently, there is a single BGP session even if there are
multiple direct connections between BGP SPF speakers. The BGP-LS-SPF
Link NLRI is advertised as long as a BGP session has been
established, the BGP-LS-SPF AFI/SAFI capability has been exchanged
[RFC4760], the link is operational as determined using liveliness
detection mechanisms, and, optionally, the EoR Marker has been
received as described in the Section 4.1. This is much like the
previous peering model only peering is between loopback addresses and
the interconnecting links can be unnumbered. However, since there
are BGP sessions between every directly-connected node in the BGP SPF
routing domain, there is only a reduction in BGP sessions when there
are parallel links between nodes. Hence, this peering model is
RECOMMENDED over the single-hop peering model Section 4.1.
Patel, et al. Expires 14 September 2023 [Page 8]
Internet-Draft BGP Link-State SPF Routing March 2023
An End-of-RIB (EoR) Marker [RFC4724] for the BGP-LS-SPF SAFI MAY also
be expected prior to advertising the BGP-LS Link NLRI for the link(s)
to this peer.
4.3. BGP Peering in Route-Reflector or Controller Topology
In this model, BGP SPF speakers peer solely with one or more Route
Reflectors [RFC4456] or controllers. As in the previous model,
direct connection discovery and liveliness detection for those links
in the BGP SPF routing domain are done outside of the BGP protocol.
BGP-LS-SPF Link NLRI is advertised as long as the corresponding link
is considered up as per the chosen liveness detection mechanism.
This peering model, known as sparse peering, allows for fewer BGP
sessions and, consequently, fewer instances of the same NLRI received
from multiple peers. Normally, the route-reflectors or controller
BGP sessions would be on directly-connected links to avoid dependence
on another routing protocol for session connectivity. However,
multi-hop peering is not precluded. The number of BGP sessions is
dependent on the redundancy requirements and the stability of the BGP
sessions. This is discussed in greater detail in
[I-D.ietf-lsvr-applicability].
The controller may use constraints to determine when to advertise
BGP-LS-SPF NLRI for BGP-LS peers. For example, a controller may
defer advertisement until the EoR marker has been received from both
BGP peers and both have received each others NLRI. These constraints
are outside the scope of this document and, since they are internal
to the controller, need not be standardized.
5. BGP Shortest Path Routing (SPF) Protocol Extensions
5.1. BGP-LS Shortest Path Routing (SPF) SAFI
This document introduces the BGP-LS-SPF SAFI with a value of 80. The
SPF-based decision process (Section 6) applies only to the BGP-LS-SPF
SAFI and MUST NOT be used with other combinations of the BGP-LS AFI
(16388). In order for two BGP SPF speakers to exchange BGP-LS-SPF
NLRI, they MUST exchange the Multiprotocol Extensions Capability
[RFC4760] to ensure that they are both capable of properly processing
such NLRI. This is done with AFI 16388 / SAFI 80. The BGP-LS-SPF
SAFI is used to advertise IPv4 and IPv6 prefix information in a
format facilitating an SPF-based decision process.
Patel, et al. Expires 14 September 2023 [Page 9]
Internet-Draft BGP Link-State SPF Routing March 2023
5.1.1. BGP-LS-SPF NLRI TLVs
All the TLVs defined for BGP-LS [I-D.ietf-idr-rfc7752bis] are
applicable and can be used with the BGP-LS-SPF SAFI to describe
links, nodes, and prefixes comprising IGP link-state information.
The NLRI and conprising TLVs MUST be processed as specified in
section 5.1 [I-D.ietf-idr-rfc7752bis]. TLVs specified as mandatory
in [I-D.ietf-idr-rfc7752bis] are considered mandatory for the BGP-LS-
SPF SAFI as well. If a mandatory TLV is not specified, the NLRI is
not used in the BGP SPF route calculation. All the other TLVs are
considered as an optional TLVs.
5.1.2. BGP-LS Attribute
The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly same format
of the BGP-LS AFI [I-D.ietf-idr-rfc7752bis]. In other words, all the
TLVs used in BGP-LS attribute of the BGP-LS AFI are applicable and
used for the BGP-LS attribute of the BGP-LS-SPF SAFI. This attribute
is an optional, non-transitive BGP attribute that is used to carry
link, node, and prefix properties and attributes. The BGP-LS
attribute is a set of TLVs.
The BGP-LS attribute may potentially grow large in size depending on
the amount of link-state information associated with a single Link-
State NLRI. The BGP specification [RFC4271] mandates a maximum BGP
message size of 4096 octets. It is RECOMMENDED that an
implementation support [RFC8654] in order to accommodate larger size
of information within the BGP-LS Attribute. BGP SPF speakers MUST
ensure that they limit the TLVs included in the BGP-LS Attribute to
ensure that a BGP update message for a single Link-State NLRI does
not cross the maximum limit for a BGP message. The determination of
the types of TLVs to be included by the BGP SPF speaker originating
the attribute is outside the scope of this document. When a BGP SPF
speaker finds that it is exceeding the maximum BGP message size due
to addition or update of some other BGP Attribute (e.g., AS_PATH), it
MUST consider the BGP-LS Attribute to be malformed and the attribute
discard handling of [RFC7606] applies.
5.2. Extensions to BGP-LS
[I-D.ietf-idr-rfc7752bis] describes a mechanism by which link-state
and TE information can be collected from IGPs and shared with
external components using the BGP protocol. It describes both the
definition of the BGP-LS NLRI that advertise links, nodes, and
prefixes comprising IGP link-state information and the definition of
a BGP path attribute (BGP-LS attribute) that carries link, node, and
prefix properties and attributes, such as the link and prefix metric
Patel, et al. Expires 14 September 2023 [Page 10]
Internet-Draft BGP Link-State SPF Routing March 2023
or auxiliary Router-IDs of nodes, etc. This document extends the
usage of BGP-LS NLRI for the purpose of BGP SPF calculation via
advertisement in the BGP-LS-SPF SAFI.
The protocol identifier specified in the Protocol-ID field
[I-D.ietf-idr-rfc7752bis] represents the origin of the advertised
NLRI. For Node NLRI and Link NLRI, this MUST be the direct protocol
(4). Node or Link NLRI with a Protocol-ID other than the direct
protocol is considered malformed. For Prefix NLRI, the specified
Protocol-ID MUST be the origin of the prefix. The local and remote
node descriptors for all NLRI MUST include the BGP Identifier (TLV
516) and the AS Number (TLV 512) [I-D.ietf-idr-rfc7752bis]. The BGP
Confederation Member (TLV 517) [I-D.ietf-idr-rfc7752bis] is currently
not applicable.
5.2.1. Node NLRI Usage
The Node NLRI MUST be advertised unconditionally by all routers in
the BGP SPF routing domain.
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Capability TLV
The SPF capability is an additional Node Attribute TLV. This
attribute TLV MUST be included with the BGP-LS-SPF SAFI and SHOULD
NOT be used for other SAFIs. The TLV type is 1180. The Node
Attribute TLV contains a single-octet SPF algorithm as defined in
[RFC8665].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1180) | Length - (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Algorithm |
+-+-+-+-+-+-+-+-+
The SPF Algorithm field is used to advertise the algorithm used by
the router to calculate the paths to other routers in the BGP SPF
routing domain. The SPF algorithm inherits the values from the IGP
Algorithm Types registry [RFC8665]. Algorithm 0, (Shortest Path
Algorithm (SPF) based on link metric, is supported and described in
Section 6.3. Support for other algorithm types is beyond the scope
of this specification.
Patel, et al. Expires 14 September 2023 [Page 11]
Internet-Draft BGP Link-State SPF Routing March 2023
When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability TLV with same SPF algorithm are
included in the SPF computation Section 6.3. An implementation MAY
optionally log detection of a BGP node that has either not advertised
the SPF capability TLV or is advertising the SPF capability TLV with
an algorithm type other than 0.
5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF Node NLRI is defined to
indicate the status of the node with respect to the BGP SPF
calculation. This is used to rapidly take a node out of service
(refer to Section 6.5.2) or to indicate the node is not to be used
for transit (i.e., non-local) traffic (refer to Section 6.3). If the
SPF Status TLV is not included with the Node NLRI, the node is
considered to be up and is available for transit traffic. The SPF
status is acted upon with the execution of the next SPF calculation
(refer to Section 6.3).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
SPF Status Values: 0 - Reserved
1 - Node Unreachable with respect to BGP SPF
2 - Node does not support transit with respect
to BGP SPF
3-254 - Undefined
255 - Reserved
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184).
If a BGP SPF speaker received the Node NLRI but the SPF Status TLV is
not received, then any previously received information is considered
as implicitly withdrawn and the update is propagated to other BGP SPF
speakers. A BGP SPF speaker receiving a BGP Update containing a SPF
Status TLV in the BGP-LS attribute [I-D.ietf-idr-rfc7752bis] with a
value that is undefined values SHOULD be advertised to other BGP SPF
speakers. However, a BGP SPF speaker MUST NOT use the Status TLV in
its SPF computation. An implementation MAY log this condition for
further analysis.
Patel, et al. Expires 14 September 2023 [Page 12]
Internet-Draft BGP Link-State SPF Routing March 2023
5.2.2. Link NLRI Usage
The criteria for advertisement of Link NLRI are discussed in
Section 4.
Link NLRI is advertised with unique local and remote node descriptors
dependent on the IP addressing. For IPv4 links, the link's local
IPv4 (TLV 259) and remote IPv4 (TLV 260) addresses are used. For
IPv6 links, the local IPv6 (TLV 261) and remote IPv6 (TLV 262)
addresses are used. For unnumbered links, the link local/remote
identifiers (TLV 258) are used. For links supporting having both
IPv4 and IPv6 addresses, both sets of descriptors MAY be included in
the same Link NLRI. The link descriptors are described in table 4 of
[I-D.ietf-idr-rfc7752bis].
For a link to be used in SPF computation for a given address family,
i.e., IPv4 or IPv6, both routers connecting the link MUST have an
address in the same subnet for that address family. However, an IPv4
or IPv6 prefix associated with the link MAY be installed without the
corresponding address on the other side of link.
The IGP metric attribute TLV (TLV 1095) MUST be advertised. If a BGP
SPF speaker receives a Link NLRI without an IGP metric attribute TLV,
then it MUST consider the received NLRI as a malformed and the
receiving BGP SPF speaker MUST handle such malformed NLRI as 'Treat-
as-withdraw' [RFC7606]. The BGP SPF metric length is 4 octets. A
metric is associated with the output side of each router interface.
This metric is configurable by the system administrator. The lower
the metric, the more likely the interface is to be used to forward
data traffic. One possible default for metric would be to give each
interface a metric of 1 making it effectively a hop count.
The usage of other link attribute TLVs is beyond the scope of this
document.
5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs
Two BGP-LS Attribute TLVs of the BGP-LS-SPF Link NLRI are defined to
advertise the prefix length associated with the IPv4 and IPv6 link
prefixes derived from the link descriptor addresses. The prefix
length is used for the optional installation of prefixes
corresponding to Link NLRI as defined in Section 6.3.
Patel, et al. Expires 14 September 2023 [Page 13]
Internet-Draft BGP Link-State SPF Routing March 2023
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPv4 (1182) or IPv6 Type (1183)| Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Length |
+-+-+-+-+-+-+-+-+
Prefix-length - A one-octet length restricted to 1-32 for IPv4
Link NLRI endpoint prefixes and 1-128 for IPv6
Link NLRI endpoint prefixes.
The Prefix-Length TLV is only relevant to Link NLRIs. When received
with any NLRIs other than Link NRLIs, the corresponding Link NLRI is
considered as malformed and MUST be handled as 'Treat-as-withdraw'
[RFC7606]. An implementation MAY log an error for further analysis.
The maximum prefix-length is 32 bits for an IPv4 Prefix-Length TLV A
prefix-length field and 128 bits for an IPv6 Prefix-Length TLV. A
prefix-length field indicating a larger value is in error and the the
corresponding Link NLRI is considered as malformed and MUST be
handled as 'Treat-as-withdraw' [RFC7606]. An implementation MAY log.
An implementation MAY log an error for further analysis.
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF Link NLRI is defined to
indicate the status of the link with respect to the BGP SPF
calculation. This is used to expedite convergence for link failures
as discussed in Section 6.5.1. If the SPF Status TLV is not included
with the Link NLRI, the link is considered up and available. The SPF
status is acted upon with the execution of the next SPF calculation
Section 6.3. A single TLV type is shared by the Node, Link, and
Prefix NLRI. The TLV type is 1184.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Link Unreachable with respect to BGP SPF
2-254 - Undefined
255 - Reserved
Patel, et al. Expires 14 September 2023 [Page 14]
Internet-Draft BGP Link-State SPF Routing March 2023
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184). This implies that a BGP Update cannot contain multiple NLRI
with differing status. If the BGP-LS-SPF Status TLV is advertised
and the advertised value is not defined, then the SPF Status TLV is
ignored and not used in SPF computation but is still advertised to
other BGP SPF speakers. An implementation MAY log an error for
further analysis.
If a BGP SPF speaker received the Link NLRI but the SPF Status TLV is
not received, then any previously received information is considered
as implicitly withdrawn and the update is propagated to other BGP SPF
speakers. A BGP SPF speaker receiving a BGP Update containing an SPF
Status TLV in the BGP-LS attribute [I-D.ietf-idr-rfc7752bis] with a
value that is undefined SHOULD be advertised to other BGP SPF
speakers. However, a BGP SPF speaker MUST NOT use the Status TLV in
its SPF computation. An implementation MAY log this information for
further analysis.
5.2.3. IPv4/IPv6 Prefix NLRI Usage
IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor and
the prefix and length. The Prefix Descriptors field includes the IP
Reachability Information TLV (TLV 265) as described in
[I-D.ietf-idr-rfc7752bis]. The Prefix Metric TLV (TLV 1155) MUST be
advertised. The IGP Route Tag TLV (TLV 1153) MAY be advertised. The
usage of other BGP-LS attribute TLVs is beyond the scope of this
document.
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV to BGP-LS-SPF Prefix NLRI is defined to
indicate the status of the prefix with respect to the BGP SPF
calculation. This is used to expedite convergence for prefix
unreachability as discussed in Section 6.5.1. If the SPF Status TLV
is not included with the Prefix NLRI, the prefix is considered
reachable. A single TLV type is shared by the Node, Link, and Prefix
NLRI. The TLV type is 1184.
Patel, et al. Expires 14 September 2023 [Page 15]
Internet-Draft BGP Link-State SPF Routing March 2023
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Prefix Unreachable with respect to SPF
2-254 - Undefined
255 - Reserved
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184). This implies that a BGP Update cannot contain multiple NLRI
with differing status. If the BGP-LS-SPF Status TLV is advertised
and the advertised value is not defined, then the SPF Status TLV is
ignored and not used in SPF computation but is still advertised to
other BGP SPF speakers. An implementation MAY log an error for
further analysis.
If a BGP SPF speaker received the Prefix NLRI but the SPF Status TLV
is not received, then any previously received information is
considered as implicitly withdrawn and the update is propagated to
other BGP SPF speakers. A BGP SPF speaker receiving a BGP Update
containing an SPF Status TLV in the BGP-LS attribute
[I-D.ietf-idr-rfc7752bis] with a value that is undefined SHOULD be
advertised to other BGP SPF speakers. However, a BGP SPF speaker
MUST NOT use the Status TLV in its SPF computation. An
implementation MAY log this information for further analysis.
5.2.4. BGP-LS Attribute Sequence-Number TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF NLRI types is defined to
assure the most recent version of a given NLRI is used in the SPF
computation. The Sequence-Number TLV is mandatory for BGP-LS-SPF
NLRI. The TLV type 1181 has been assigned by IANA. The BGP-LS
Attribute TLV contains an 8-octet sequence number. The usage of the
Sequence Number TLV is described in Section 6.1.
Patel, et al. Expires 14 September 2023 [Page 16]
Internet-Draft BGP Link-State SPF Routing March 2023
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1181) | Length (8 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number The 64-bit strictly-increasing sequence number MUST
be incremented for every self-originated version of BGP-LS-SPF NLRI.
BGP SPF speakers implementing this specification MUST use available
mechanisms to preserve the sequence number's strictly increasing
property for the deployed life of the BGP SPF speaker (including cold
restarts). One mechanism for accomplishing this would be to use the
high-order 32 bits of the sequence number as a wrap/boot count that
is incremented any time the BGP router loses its sequence number
state or the low-order 32 bits wrap.
When incrementing the sequence number for each self-originated NLRI,
the sequence number should be treated as an unsigned 64-bit value.
If the lower-order 32-bit value wraps, the higher-order 32-bit value
should be incremented and saved in non-volatile storage. If a BGP
SPF speaker completely loses its sequence number state (e.g., the BGP
SPF speaker hardware is replaced or experiences a cold-start), the
BGP NLRI selection rules (see Section 6.1) insure convergence, albeit
not immediately.
If the Sequence-Number TLV is not received then the corresponding
NLRI is considered as malformed and MUST be handled as 'Treat-as-
withdraw'. An implementation MAY log an error for further analysis.
5.3. NEXT_HOP Attribute Manipulation
All BGP peers that support SPF extensions would locally compute the
Local-RIB Next-Hop as a result of the SPF process. Consequently, the
Next Hop Attribute is always ignored on receipt. The Next Hop
address MUST be encoded as described in [RFC4760]. BGP SPF speakers
MUST interpret the Next Hop address of MP_REACH_NLRI attribute as an
IPv4 address whenever the length of the Next Hop address is 4 octets,
and as a IPv6 address whenever the length of the Next Hop address is
16 octets.
Patel, et al. Expires 14 September 2023 [Page 17]
Internet-Draft BGP Link-State SPF Routing March 2023
[RFC4760] modifies the rules of NEXT_HOP attribute whenever the
multiprotocol extensions for BGP-4 are enabled. BGP SPF speakers
MUST set the NEXT_HOP attribute according to the rules specified in
[RFC4760] as the BGP-LS-SPF routing information is carried within the
multiprotocol extensions for BGP-4.
6. Decision Process with SPF Algorithm
The Decision Process described in [RFC4271] takes place in three
distinct phases. The Phase 1 decision function of the Decision
Process is responsible for calculating the degree of preference for
each route received from a BGP SPF speaker's peer. The Phase 2
decision function is invoked on completion of the Phase 1 decision
function and is responsible for choosing the best route out of all
those available for each distinct destination, and for installing
each chosen route into the Local-RIB. The combination of the Phase 1
and 2 decision functions is characterized as a Path Vector algorithm.
The SPF based Decision process replaces the BGP Decision process
described in [RFC4271]. This process starts with selecting only
those Node NLRI whose SPF capability TLV matches with the local BGP
SPF speaker's SPF capability TLV value. Since Link-State NLRI always
contains the local node descriptor Section 5.2, each NLRI is uniquely
originated by a single BGP SPF speaker in the BGP SPF routing domain
(the BGP node matching the NLRI's Node Descriptors). Instances of
the same NLRI originated by multiple BGP SPF speakers would be
indicative of a configuration error or a masquerading attack (refer
to Section 9). These selected Node NLRI and their Link/Prefix NLRI
are used to build a directed graph during the SPF computation as
described below. The best routes for BGP prefixes are installed in
the RIB as a result of the SPF process.
When BGP-LS-SPF NLRI is received, all that is required is to
determine whether it is the most recent by examining the Node-ID and
sequence number as described in Section 6.1. If the received NLRI
has changed, it is advertised to other BGP-LS-SPF peers. If the
attributes have changed (other than the sequence number), a BGP SPF
calculation is triggered. However, a changed NLRI MAY be advertised
immediately to other peers and prior to any SPF calculation. Note
that the BGP MinRouteAdvertisementIntervalTimer and
MinASOriginationIntervalTimer [RFC4271] timers are not applicable to
the BGP-LS-SPF SAFI. The scheduling of the SPF calculation, as
described in Section 6.3, is an implementation issue. Scheduling MAY
be dampened consistent with the SPF back-off algorithm specified in
[RFC8405].
Patel, et al. Expires 14 September 2023 [Page 18]
Internet-Draft BGP Link-State SPF Routing March 2023
The Phase 3 decision function of the Decision Process [RFC4271] is
also simplified since under normal SPF operation, a BGP SPF speaker
MUST advertise the changed NLRIs to all BGP peers with the BGP-LS-SPF
AFI/SAFI and install the changed routes in the GLOBAL-RIB. The only
exception are unchanged NLRIs or stale NLRIs, i.e., NLRI received
with a less recent (numerically smaller) sequence number.
6.1. BGP SPF NLRI Selection
The rules for all BGP-LS-SPF NLRIs selection for phase 1 of the BGP
decision process, section 9.1.1 [RFC4271], no longer apply.
1. NLRI originated by directly connected BGP SPF peers are
preferred. This condition can be determined by comparing the BGP
Identifiers in the received Local Node Descriptor and OPEN
message. This rule assures that stale NLRI is updated even if a
BGP-LS router loses its sequence number state due to a cold-
start.
2. The NLRI with the most recent Sequence Number TLV, i.e., highest
sequence number is selected.
3. The NLRI received from the BGP SPF speaker with the numerically
larger BGP Identifier is preferred.
When a BGP SPF speaker completely loses its sequence number state,
i.e., due to a cold start, or in the unlikely possibility that 64-bit
sequence number wraps, the BGP routing domain will still converge.
This is due to the fact that BGP SPF speakers adjacent to the router
always accepts self-originated NLRI from the associated speaker as
more recent (rule # 1). When a BGP SPF speaker reestablishes a
connection with its peers, any existing sessions are taken down and
stale NLRI are replaced. The adjacent BGP SPF speakers update their
NLRI advertisements and advertise to their neighbors until the BGP
routing domain has converged.
The modified SPF Decision Process performs an SPF calculation rooted
at the local BGP SPF speaker using the metrics from the Link
Attribute IGP Metric TLV (1095) and the Prefix Attribute Prefix
Metric TLV (1155) [I-D.ietf-idr-rfc7752bis]. As a result, any other
BGP attributes that would influence the BGP decision process defined
in [RFC4271] including ORIGIN, MULTI_EXIT_DISC, and LOCAL_PREF
attributes are ignored by the SPF algorithm. The NEXT_HOP attribute
is discussed in Section 5.3. The AS_PATH and AS4_PATH [RFC6793]
attributes are preserved and used for loop detection [RFC4271]. They
are ignored during the SPF computation for BGP-LS-SPF NRLIs.
Patel, et al. Expires 14 September 2023 [Page 19]
Internet-Draft BGP Link-State SPF Routing March 2023
6.1.1. BGP Self-Originated NLRI
Node, Link, or Prefix NLRI with Node Descriptors matching the local
BGP SPF speaker are considered self-originated. When self-originated
NLRI is received and it doesn't match the local node's NLRI content
(including sequence number), special processing is required.
* If self-originated NLRI is received and the sequence number is
more recent (i.e., greater than the local node's sequence number
for the NLRI), the NLRI sequence number is advanced to one greater
than the received sequence number and the NLRI is readvertised to
all peers.
* If self-originated NLRI is received and the sequence number is the
same as the local node's sequence number but the attributes
differ, the NLRI sequence number is advanced to one greater than
the received sequence number and the NLRI is readvertised to all
peers.
* If self-originated Link or Prefix NLRI is received and the Link or
Prefix NLRI is no longer being advertised by the local node, the
NLRI is withdrawn.
The above actions are performed immediately when the first instance
of a newer self-originated NLRI is received. In this case, the newer
instance is considered to be a stale instance that was advertised by
the local node prior to a restart where the NLRI state is lost.
However, if subsequent newer self-originated NLRI is received for the
same Node, Link, or Prefix NLRI, the readvertisement or withdrawal is
delayed by 5 seconds since it is likely being advertised by a
misconfigured or rogue BGP SPF speaker (refer to Section 9).
6.2. Dual Stack Support
The SPF-based decision process operates on Node, Link, and Prefix
NLRIs that support both IPv4 and IPv6 addresses. Whether to run a
single SPF computation or multiple SPF computations for separate AFs
is an implementation matter. Normally, IPv4 next-hops are calculated
for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
prefixes.
Patel, et al. Expires 14 September 2023 [Page 20]
Internet-Draft BGP Link-State SPF Routing March 2023
6.3. SPF Calculation based on BGP-LS-SPF NLRI
This section details the BGP-LS-SPF local routing information base
(RIB) calculation. The router uses BGP-LS-SPF Node, Link, and Prefix
NLRI to compute routes using the following algorithm. This
calculation yields the set of routes associated with the BGP SPF
Routing Domain. A router calculates the shortest-path tree using
itself as the root. Optimizations to the BGP-LS-SPF algorithm are
possible but MUST yield the same set of routes. The algorithm below
supports Equal Cost Multi-Path (ECMP) routes. Weighted Unequal Cost
Multi-Path routes are out of scope.
The following abstract data structures are defined in order to
specify the algorithm.
* Local Route Information Base (Local-RIB) - This routing table
contains reachability information (i.e., next hops) for all
prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
reachability. Implementations may choose to implement this with
separate RIBs for each address family and/or Prefix versus Node
reachability.
* Global Routing Information Base (GLOBAL-RIB) - This is the Routing
Information Base (RIB) containing the current routes that are
installed in the router's forwarding plane. This is commonly
referred to in networking parlance as "the RIB".
* Link State NLRI Database (LSNDB) - Database of BGP-LS-SPF NLRI
that facilitates access to all Node, Link, and Prefix NLRI.
* Candidate List (CAN-LIST) - This is a list of candidate Node NLRIs
used during the BGP SPF calculation. The list is sorted by the
cost to reach the Node NLRI with the Node NLRI with the lowest
reachability cost at the head of the list. This facilitates
execution of the Dijkstra algorithm where the shortest paths
between the local node and other nodes in graph area computed.
The CAN-LIST is typically implemented as a heap but other data
structures have been used.
The algorithm is comprised of the steps below:
1. The current Local-RIB is invalidated, and the CAN-LIST is
initialized to empty. The Local-RIB is rebuilt during the course
of the SPF computation. The existing routing entries are
preserved for comparison to determine changes that need to be
made to the GLOBAL-RIB in step 6.
Patel, et al. Expires 14 September 2023 [Page 21]
Internet-Draft BGP Link-State SPF Routing March 2023
2. The computing router's Node NLRI is updated in the Local-RIB with
a cost of 0 and the Node NLRI is also added to the CAN-LIST. The
next-hop list is set to the internal loopback next-hop.
3. The Node NLRI with the lowest cost is removed from the CAN-LIST
for processing. If the BGP-LS Node attribute includes an SPF
Status TLV (refer to Section 5.2.1.2) indicating the node is
unreachable, the Node NLRI is ignored and the next lowest cost
Node NLRI is selected from the CAN-LIST. The Node corresponding
to this NLRI is referred to as the Current-Node. If the CAN-LIST
list is empty, the SPF calculation has completed and the
algorithm proceeds to step 6.
4. All the Prefix NLRI with the same Local Node Descriptors as the
Current-Node is considered for installation. The next-hop(s) for
these Prefix NLRI are inherited from the Current-Node. If the
Current-Node is for the local BGP Router, the next-hop for the
prefix is a direct next-hop. The cost for each prefix is the
metric advertised in the Prefix Attribute Prefix Metric TLV
(1155) added to the cost to reach the Current-Node. The
following is done for each Prefix NLRI (referred to as the
Current-Prefix):
* If the BGP-LS Prefix attribute includes an SPF Status TLV
indicating the prefix is unreachable, the Current-Prefix is
considered unreachable and the next Prefix NLRI is examined in
Step 4.
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the Local-RIB metric is less than the Current-Prefix's
metric, the Current-Prefix does not contribute to the route
and the next Prefix NLRI is examined in Step 4.
* If the Current-Prefix's corresponding prefix is not in the
Local-RIB, the prefix is installed with the Current-Node's
next-hops installed as the Local-RIB route's next-hops and the
metric being updated. If the IGP Route Tag TLV (1153) is
included in the Current-Prefix's NLRI Attribute, the tag(s)
are installed in the current Local-RIB route's tag(s).
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the cost is less than the Local-RIB route's metric,
the prefix is installed with the Current-Node's next-hops
replacing the Local-RIB route's next-hops and the metric being
updated and any route tags removed. If the IGP Route Tag TLV
(1153) is included in the Current-Prefix's NLRI Attribute, the
tag(s) are installed in the current Local-RIB route's tag(s).
Patel, et al. Expires 14 September 2023 [Page 22]
Internet-Draft BGP Link-State SPF Routing March 2023
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the cost is the same as the Local-RIB route's metric,
the Current-Node's next-hops are merged with Local-RIB route's
next-hops. The algorithm below supports Equal Cost Multi-Path
(ECMP) routes. Some platforms or implementations may have
limits on the number of ECMP routes that can be supported.
The setting or identification of any limitations is outside
the scope if this document. Nonetheless, step 4 (below)
includes a set of recommendations in case such as limit is
encountered. Weighted Unequal Cost Multi-Path routes are out
of scope as well.
5. All the Link NLRI with the same Node Identifiers as the Current-
Node is considered for installation. Each link is examined and
is referred to in the following text as the Current-Link. The
cost of the Current-Link is the advertised IGP Metric TLV (1095)
from the Link NLRI BGP-LS attribute added to the cost to reach
the Current-Node. If the Current-Node is for the local BGP
Router, the next-hop for the link is a direct next-hop pointing
to the corresponding local interface. For any other Current-
Node, the next-hop(s) for the Current-Link is inherited from the
Current-Node. The following is done for each link:
a. The prefix(es) associated with the Current-Link are installed
into the Local-RIB using the same rules as were used for
Prefix NLRI in the previous steps. Optionally, in
deployments where BGP-SPF routers have limited routing table
capacity, installation of these subnets can be suppressed.
Suppression has an operational impact as the IPv4/IPv6 link
endpoint addresses is not be reachable and tools such as
traceroute will display addresses that are not reachable.
b. If the Current-Node NLRI attributes includes the SPF Status
TLV (refer to Section 5.2.1.2) and the status indicates that
the Node doesn't support transit, the next link for the
Current-Node is processed in Step 5.
c. If the Current-Link's NLRI attribute includes an SPF Status
TLV indicating the link is down, the BGP-LS-SPF Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.
d. The Current-Link's Remote Node NLRI is accessed (i.e., the
Node NLRI with the same Node identifiers as the Current-
Link's Remote Node Descriptors). If it exists, it is
referred to as the Remote-Node and the algorithm proceeds as
follows:
Patel, et al. Expires 14 September 2023 [Page 23]
Internet-Draft BGP Link-State SPF Routing March 2023
* If the Remote-Node's NLRI attribute includes an SPF Status
TLV indicating the node is unreachable, the next link for
the Current-Node is examined in Step 5.
* All the Link NLRI corresponding the Remote-Node are
searched for a Link NLRI pointing to the Current-Node.
Each Link NLRI is examined for Remote Node Descriptors
matching the Current-Node and Link Descriptors matching
the Current-Link. For numbered links to match, the Link
Descriptors MUST share a common IPv4 or IPv6 subnet. For
unnumbered links to match, the Current Link's Local
Identifier MUST match the Remote Node Link's Remote
Identifier and the Current Link's Remote Identifier MUST
the Remote Node Link's Local Identifier [RFC5307]. If
these conditions are satisfied for one of the Remote-
Node's links, the bi-directional connectivity check
succeeds and the Remote-Node may be processed further.
The Remote-Node's Link NLRI providing bi-directional
connectivity is referred to as the Remote-Link. If no
Remote-Link is found, the next link for the Current-Node
is examined in Step 5.
* If the Remote-Link NLRI attribute includes an SPF Status
TLV indicating the link is down, the Remote-Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.
* If the Remote-Node is not on the CAN-LIST, it is inserted
based on the cost. The Remote Node's cost is the cost of
Current-Node added the Current-Link's IGP Metric TLV
(1095). The next-hop(s) for the Remote-Node are inherited
from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with a
higher cost, it must be removed and reinserted with the
Remote-Node cost based on the Current-Link (as calculated
in the previous step). The next-hop(s) for the Remote-
Node are inherited from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with
the same cost, it need not be reinserted on the CAN-LIST.
However, the Current-Link's next-hop(s) must be merged
into the current set of next-hops for the Remote-Node.
* If the Remote-Node NLRI is already on the CAN-LIST with a
lower cost, it need not be reinserted on the CAN-LIST.
Patel, et al. Expires 14 September 2023 [Page 24]
Internet-Draft BGP Link-State SPF Routing March 2023
e. Return to step 3 to process the next lowest cost Node NLRI on
the CAN-LIST.
6. The Local-RIB is examined and changes (adds, deletes,
modifications) are installed into the GLOBAL-RIB. For each route
in the Local-RIB:
* If the route was added during the current BGP SPF computation,
install the route into the GLOBAL-RIB.
* If the route modified during the current BGP SPF computation
(e.g., metric, tags, or next-hops), update the route in the
GLOBAL-RIB.
* If the route was not installed during the current BGP SPF
computation, remove the route from both the GLOBAL-RIB and the
Local-RIB.
6.4. IPv4/IPv6 Unicast Address Family Interaction
While the BGP-LS-SPF address family and the IPv4/IPv6 unicast address
families MAY install routes into the same device routing tables, they
operate independently much the same as multiple IGPs would operate
today (i.e., "Ships-in-the-Night" mode). There is no implicit route
redistribution between the BGP address families.
It is RECOMMENDED that BGP-LS-SPF IPv4/IPv6 route computation and
installation be given scheduling priority by default over other BGP
address families as these address families are considered as underlay
SAFIs. Similarly, it is RECOMMENDED that the route preference or
administrative distance give active route installation preference to
BGP-LS-SPF IPv4/IPv6 routes over BGP routes from other AFI/SAFIs.
However, this preference MAY be overridden by an operator-configured
policy.
6.5. NLRI Advertisement
6.5.1. Link/Prefix Failure Convergence
A local failure prevents a link from being used in the SPF
calculation due to the IGP bi-directional connectivity requirement.
Consequently, local link failures SHOULD always be given priority
over updates (e.g., withdrawing all routes learned on a session) in
order to ensure the highest priority propagation and optimal
convergence.
Patel, et al. Expires 14 September 2023 [Page 25]
Internet-Draft BGP Link-State SPF Routing March 2023
An IGP stops using the link as soon as the link-state update for one
side of the link is received. With a BGP advertisement, the link
would continue to be used until the last copy of the BGP-LS-SPF Link
NLRI is withdrawn. In order to avoid this delay, the originator of
the Link NLRI SHOULD advertise a more recent version with an
increased Sequence Number TLV for the BGP-LS-SPF Link NLRI including
the SPF Status TLV (refer to Section 5.2.2.2) indicating the link is
down with respect to BGP SPF. The configurable
LinkStatusDownAdvertise timer controls the interval that the BGP-LS-
LINK NLRI is advertised with SPF Status indicating the link is down
prior to withdrawal. If the link becomes available in that period,
the originator of the BGP-LS-SPF LINK NLRI SHOULD advertise a more
recent version of the BGP-LS-SPF Link NLRI without the SPF Status TLV
in the BGP-LS Link Attributes. The suggested default value for the
LinkStatusDownAdvertise timer is 2 seconds.
Similarly, when a prefix becomes unreachable, a more recent version
of the BGP-LS-SPF Prefix NLRI SHOULD be advertised with the SPF
Status TLV (refer to Section 5.2.3.1) indicating the prefix is
unreachable in the BGP-LS Prefix Attributes and the prefix will be
considered unreachable with respect to BGP SPF. The configurable
PrefixStatusDownAdvertise timer controls the interval that the BGP-
LS-Prefix NLRI is advertised with SPF Status indicating the prefix is
unreachable prior to withdrawal. If the prefix becomes reachable in
that period, the originator of the BGP-LS-SPF Prefix NLRI SHOULD
advertise a more recent version of the BGP-LS-SPF Prefix NLRI without
the SPF Status TLV in the BGP-LS Prefix Attributes. The suggested
default value for the PrefixStatusDownAdvertise timer is 2 seconds.
6.5.2. Node Failure Convergence
With BGP without graceful restart [RFC4724], all the NLRI advertised
by a node are implicitly withdrawn when a session failure is
detected. If fast failure detection such as BFD is utilized, and the
node is on the fastest converging path, the most recent versions of
BGP-LS-SPF NLRI may be withdrawn. This results in an older version
of the NLRI being used until the new versions arrive and,
potentially, unnecessary route flaps. For the BGP-LS-SPF SAFI, NLRI
SHOULD NOT be implicitly withdrawn immediately to prevent such
unnecessary route flaps. The configurable
NLRIImplicitWithdrawalDelay timer controls the interval that NLRI is
retained prior to implicit withdrawal after a BGP SPF speaker has
transitioned out of Established state. This does delay convergence
since the adjacent nodes detect the link failure and advertise a more
recent NLRI indicating the link is down with respect to BGP SPF
(refer to Section 6.5.1) and the bi-directional connectivity check
fails during the BGP SPF calculation (refer to Section 6.3). The
suggested default value for the NLRIImplicitWithdrawalDelay timer is
Patel, et al. Expires 14 September 2023 [Page 26]
Internet-Draft BGP Link-State SPF Routing March 2023
2 seconds.
7. Error Handling
This section describes the Error Handling actions, as described in
[RFC7606], that are specific to SAFI BGP-LS-SPF BGP Update message
processing.
7.1. Processing of BGP-LS-SPF TLVs
When a BGP SPF speaker receives a BGP Update containing a malformed
Node NLRI SPF Status TLV in the BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and MUST
NOT pass it to other BGP peers as specified in [RFC7606]. When
discarding an associated Node NLRI with a malformed TLV, a BGP SPF
speaker SHOULD log an error for further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
Link NLRI SPF Status TLV in the BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and MUST
NOT pass it to other BGP peers as specified in [RFC7606]. When
discarding an associated Link NLRI with a malformed TLV, a BGP SPF
speaker SHOULD log an error for further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
Prefix NLRI SPF Status TLV in the BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and MUST
NOT pass it to other BGP peers as specified in [RFC7606]. When
discarding an associated Prefix NLRI with a malformed TLV, a BGP SPF
speaker SHOULD log an error for further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
SPF Capability TLV in the Node NLRI BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and the
Node NLRI and MUST NOT pass it to other BGP peers as specified in
[RFC7606]. When discarding a Node NLRI with a malformed TLV, a BGP
SPF speaker SHOULD log an error for further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
IPv4 Prefix-Length TLV in the Link NLRI BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and the
Node NLRI and MUST NOT pass it to other BGP peers as specified in
[RFC7606]. The corresponding Link NLRI is considered as malformed
and MUST be handled as 'Treat-as-withdraw'. An implementation MAY
log an error for further analysis.
Patel, et al. Expires 14 September 2023 [Page 27]
Internet-Draft BGP Link-State SPF Routing March 2023
When a BGP SPF speaker receives a BGP Update containing a malformed
IPv6 Prefix-Length TLV in the Link NLRI BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and the
Node NLRI and MUST NOT pass it to other BGP peers as specified in
[RFC7606]. The corresponding Link NLRI is considered as malformed
and MUST be handled as 'Treat-as-withdraw'. An implementation MAY
log an error for further analysis.
7.2. Processing of BGP-LS-SPF NLRIs
A Link-State NLRI MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS-SPF NLRI to determine if it is malformed.
1. Does the sum of all TLVs found in the BGP MP_REACH_NLRI attribute
correspond to the BGP MP_REACH_NLRI length?
2. Does the sum of all TLVs found in the BGP MP_UNREACH_NLRI
attribute correspond to the BGP MP_UNREACH_NLRI length?
3. Does the sum of all TLVs found in a BGP-LS-SPF NLRI correspond to
the Total NLRI Length field of all its Descriptors?
4. When an NLRI TLV is recognized, is the length of the TLV and its
sub-TLVs valid?
5. Has the syntactic correctness of the NLRI fields been verified as
per [RFC7606]?
6. Has the rule regarding ordering of TLVs been followed as
described in Section 5.1.1?
When the error determined allows for the router to skip the malformed
NLRI(s) and continue processing of the rest of the update message
(e.g., when the TLV ordering rule is violated), then it MUST handle
such malformed NLRIs as 'Treat-as-withdraw'. In other cases, where
the error in the NLRI encoding results in the inability to process
the BGP update message (e.g., length related encoding errors), then
the router SHOULD handle such malformed NLRIs as 'AFI/SAFI disable'
when other AFI/SAFI besides BGP-LS are being advertised over the same
session. Alternately, the router MUST perform 'session reset' when
the session is only being used for BGP-LS-SPF or when its 'AFI/SAFI
disable' action is not possible.
Patel, et al. Expires 14 September 2023 [Page 28]
Internet-Draft BGP Link-State SPF Routing March 2023
In common deployment scenarios, the unicast routes installed during
BGP-LS-SPF AFI/SAFI SPF computation serve as the underlay for other
BGP AFI/SAFIs. To avoid errors encountered in other AFI/SAFIs from
impacting the BGP-LS-SPF AFI/SAFI or vice-versa, isolation mechanisms
such as separate BGP instances or separate BGP sessions (e.g., using
different addresses for peering) for BGP SPF Link-State information
distribution SHOULD be used.
7.3. Processing of BGP-LS Attribute
A BGP-LS Attribute MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS Attribute to determine if it is malformed.
1. Does the sum of all TLVs found in the BGP-LS-SPF Attribute
correspond to the BGP-LS Attribute length?
2. Has the syntactic correctness of the Attributes (including BGP-LS
Attribute) been verified as per [RFC7606]?
3. Is the length of each TLV and, when the TLV is recognized then,
its sub-TLVs in the BGP-LS Attribute valid?
When the detected error allows for the router to skip the malformed
BGP-LS Attribute and continue processing of the rest of the update
message (e.g., when the BGP-LS Attribute length and the total Path
Attribute Length are correct but some TLV/sub-TLV length within the
BGP-LS Attribute is invalid), then it MUST handle such malformed BGP-
LS Attribute as 'Attribute Discard'. In other cases, when the error
in the BGP-LS Attribute encoding results in the inability to process
the BGP update message, then the handling is the same as described
above for malformed NLRI.
Note that the 'Attribute Discard' action results in the loss of all
TLVs in the BGP-LS Attribute and not the removal of a specific
malformed TLV. The removal of specific malformed TLVs may give a
wrong indication to a BGP SPF speaker that the specific information
is being deleted or is not available.
When a BGP SPF speaker receives an update message with Link-State
NLRI(s) in the MP_REACH_NLRI but without the BGP-LS-SPF Attribute, it
is most likely an indication that a BGP SPF speaker preceding it has
performed the 'Attribute Discard' fault handling. An implementation
SHOULD preserve and propagate the Link-State NLRIs in such an update
Patel, et al. Expires 14 September 2023 [Page 29]
Internet-Draft BGP Link-State SPF Routing March 2023
message so that the BGP SPF speaker can detect the loss of link-state
information for that object and not assume its deletion/withdrawal.
This also makes it possible for a network operator to trace back to
the BGP SPF speaker which actually detected a problem with the BGP-LS
Attribute.
An implementation SHOULD log an error for further analysis for
problems detected during syntax validation.
When a BGP SPF speaker receives a BGP Update containing a malformed
IGP metric TLV in the Link NLRI BGP-LS Attribute
[I-D.ietf-idr-rfc7752bis], it MUST ignore the received TLV and the
Link NLRI and MUST NOT pass it to other BGP peers as specified in
[RFC7606]. When discarding a Link NLRI with a malformed TLV, a BGP
SPF speaker SHOULD log an error for further analysis.
8. IANA Considerations
This document defines the use of SAFI (80) for BGP SPF operation
(refer to Section 5.1), and requests IANA to assign the value from
the First Come First Serve (FCFS) range in the Subsequent Address
Family Identifiers (SAFI) Parameters registry.
This document also defines five attribute TLVs of BGP-LS-SPF NLRI.
We request IANA to assign types for the SPF capability TLV, Sequence
Number TLV, IPv4 Link Prefix-Length TLV, IPv6 Link Prefix-Length TLV,
and SPF Status TLV from the "BGP-LS Node Descriptor, Link Descriptor,
Prefix Descriptor, and Attribute TLVs" Registry.
+=========================+=================+====================+
| Attribute TLV | Suggested Value | NLRI Applicability |
+=========================+=================+====================+
| SPF Capability | 1180 | Node |
+-------------------------+-----------------+--------------------+
| SPF Status | 1184 | Node, Link, Prefix |
+-------------------------+-----------------+--------------------+
| IPv4 Link Prefix Length | 1182 | Link |
+-------------------------+-----------------+--------------------+
| IPv6 Link Prefix Length | 1183 | Link |
+-------------------------+-----------------+--------------------+
| Sequence Number | 1181 | Node, Link, Prefix |
+-------------------------+-----------------+--------------------+
Table 1: NLRI Attribute TLVs
Patel, et al. Expires 14 September 2023 [Page 30]
Internet-Draft BGP Link-State SPF Routing March 2023
9. Security Considerations
This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This
document does not change the underlying security issues inherent in
the BGP protocol [RFC4271]. The Security Considerations discussed in
[RFC4271] apply to the BGP SPF functionality as well. The analysis
of the security issues for BGP mentioned in [RFC4272] and [RFC6952]
also applies to this document. The analysis of Generic Threats to
Routing Protocols done in [RFC4593] is also worth noting. As the
modifications described in this document for BGP SPF apply to IPv4
Unicast and IPv6 Unicast as underlay SAFIs in a single BGP SPF
Routing Domain, the BGP security solutions described in [RFC6811] and
[RFC8205] are somewhat constricted as they are meant to apply for
inter-domain BGP where multiple BGP Routing Domains are typically
involved. The BGP-LS-SPF SAFI NLRI described in this document are
typically advertised between EBGP or IBGP speakers under a single
administrative domain.
The BGP SPF protocol and the BGP-LS-SPF SAFI inherit the encoding
from BGP-LS [I-D.ietf-idr-rfc7752bis], and consequently, inherit the
security considerations for BGP-LS. Additionally, given that the BGP
SPF protocol is used to install IPv4 and IPv6 Unicast routes, the BGP
SPF protocol is vulnerable to attacks to the routing control plane
that aren't applicable to BGP-LS. One notable Denial-of-Service
attack, would be to include malformed BGP attributes in a replicated
BGP Update, causing the receiving peer to treat the advertised BGP-
LS-SPF to a withdrawal [RFC7606].
In the context of the BGP peering associated with this document, a
BGP speaker MUST NOT accept updates from a peer that is not within
any administrative control of an operator. That is, a participating
BGP speaker SHOULD be aware of the nature of its peering
relationships. Such protection can be achieved by manual
configuration of peers at the BGP speaker.
In order to mitigate the risk of peering with BGP speakers
masquerading as legitimate authorized BGP speakers, it is recommended
that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
authenticate BGP sessions. If an authorized BGP peer is compromised,
that BGP peer could advertise modified Node, Link, or Prefix NLRI
which result in misrouting, repeating origination of NLRI, and/or
excessive SPF calculations. When a BGP speaker detects that its
self-originated NLRI is being originated by another BGP speaker, an
appropriate error should be logged so that the operator can take
corrective action.
Patel, et al. Expires 14 September 2023 [Page 31]
Internet-Draft BGP Link-State SPF Routing March 2023
10. Management Considerations
This section includes unique management considerations for the BGP-
LS-SPF address family.
10.1. Configuration
All routers in BGP SPF Routing Domain are under a single
administrative domain allowing for consistent configuration.
10.2. SPF Algorithm Consistency
Within a BGP SPF Routing Domain, all routers MUST use the same SPF
algorithm (refer to Section 5.2.1.1). This is the responsibility of
the administration for the routing domain.
10.3. Link Metric Configuration
For loopback prefixes, it is RECOMMMENDED that the metric be 0. For
non-loopback prefixes, the setting of the metric is a local matter
and beyond the scope of this document.
Algorithms such as setting the metric inversely to the link speed as
supported in some IGP implementations MAY be supported. However, the
details of how the metric is computed are beyond the scope of this
document.
Within a BGP SPF Routing Domain, the IGP metrics for all advertised
links SHOULD be configured or defaulted consistently. For example,
if a default metric is used for one router's links, then a similar
metric should be used for all router's links. Similarly, if the link
metric is derived from using the inverse of the link bandwidth on one
router, then this SHOULD be done for all routers and the same
reference bandwidth should be used to derive the inversely
proportional metric. Failure to do so will result in incorrect
routing based on link metric.
10.4. Adjacency End-of-RIB (EOR) Marker Requirement
Depending of the peering model, topology, and convergence
requirements, an End-of-RIB (EoR) Marker marker [RFC4724] for the
BGP-LS-SPF SAFI MAY be required from the peer prior to advertising a
BGP-LS Link NLRI for the peer. If configuration is supported, this
SHOULD be configurable at the BGP SPF instance level and SHOULD be
configured consistently throughout the BGP SPF routing domain.
Patel, et al. Expires 14 September 2023 [Page 32]
Internet-Draft BGP Link-State SPF Routing March 2023
10.5. backoff-config
In addition to configuration of the BGP-LS-SPF address family,
implementations SHOULD support the "Shortest Path First (SPF) Back-
Off Delay Algorithm for Link-State IGPs" [RFC8405]. If supported,
configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be
supported [RFC8405]. Section 6 of [RFC8405] recommends consistent
configuration of these values throughout the IGP routing domain and
this also applies to the BGP SPF Routing Domain.
10.6. Operational Data
In order to troubleshoot SPF issues, implementations SHOULD support
an SPF log including entries for previous SPF computations. Each SPF
log entry would include the BGP-LS-SPF NLRI SPF triggering the SPF,
SPF scheduled time, SPF start time, SPF end time, and SPF type if
different types of SPF are supported. Since the size of the log is
finite, implementations SHOULD also maintain counters for the total
number of SPF computations and the total number of SPF triggering
events. Additionally, to troubleshoot SPF scheduling and back-off
[RFC8405], the current SPF back-off state, remaining time-to-learn,
remaining holddown, last trigger event time, last SPF time, and next
SPF time should be available.
11. Implementation Status
Note RFC Editor: Please remove this section and the associated
references prior to publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
Patel, et al. Expires 14 September 2023 [Page 33]
Internet-Draft BGP Link-State SPF Routing March 2023
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
The BGP-LS-SPF implementation status is documented in
[I-D.psarkar-lsvr-bgp-spf-impl].
12. Acknowledgements
The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, Matt Anderson, Fred Baker, Lukas Krattiger,
Yingzhen Qu, and Haibo Wang for their review and comments. Thanks to
Pushpasis Sarkar for discussions on preventing a BGP SPF Router from
being used for non-local traffic (i.e., transit traffic).
The authors extend special thanks to Eric Rosen for fruitful
discussions on BGP-LS-SPF convergence as compared to IGPs.
13. Contributors
In addition to the authors listed on the front page, the following
co-authors have contributed to the document.
Derek Yeung
Arrcus, Inc.
derek@arrcus.com
Gunter Van De Velde
Nokia
gunter.van_de_velde@nokia.com
Abhay Roy
Arrcus, Inc.
abhay@arrcus.com
Venu Venugopal
Cisco Systems
venuv@cisco.com
Chaitanya Yadlapalli
AT&T
cy098d@att.com
14. References
Patel, et al. Expires 14 September 2023 [Page 34]
Internet-Draft BGP Link-State SPF Routing March 2023
14.1. Normative References
[I-D.ietf-idr-rfc7752bis]
Talaulikar, K., "Distribution of Link-State and Traffic
Engineering Information Using BGP", Work in Progress,
Internet-Draft, draft-ietf-idr-rfc7752bis-16, 20 February
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
idr-rfc7752bis-16>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Patel, et al. Expires 14 September 2023 [Page 35]
Internet-Draft BGP Link-State SPF Routing March 2023
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>.
[RFC8654] Bush, R., Patel, K., and D. Ward, "Extended Message
Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October
2019, <https://www.rfc-editor.org/info/rfc8654>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
14.2. Informational References
[I-D.ietf-lsvr-applicability]
Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
Applicability of Link State Vector Routing in Data
Centers", Work in Progress, Internet-Draft, draft-ietf-
lsvr-applicability-09, 20 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsvr-
applicability-09>.
[I-D.psarkar-lsvr-bgp-spf-impl]
Sarkar, P., Patel, K., Pallagatti, S., and
sajibasil@gmail.com, "BGP Shortest Path Routing Extension
Implementation Report", Work in Progress, Internet-Draft,
draft-psarkar-lsvr-bgp-spf-impl-00, 2 June 2020,
<https://datatracker.ietf.org/doc/html/draft-psarkar-lsvr-
bgp-spf-impl-00>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
Patel, et al. Expires 14 September 2023 [Page 36]
Internet-Draft BGP Link-State SPF Routing March 2023
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, DOI 10.17487/RFC4593,
October 2006, <https://www.rfc-editor.org/info/rfc4593>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Authors' Addresses
Keyur Patel
Arrcus, Inc.
Patel, et al. Expires 14 September 2023 [Page 37]
Internet-Draft BGP Link-State SPF Routing March 2023
Email: keyur@arrcus.com
Acee Lindem
301 Midenhall Way
Cary, NC 27513
United States of America
Email: acee.ietf@gmail.com
Shawn Zandi
LinkedIn
222 2nd Street
San Francisco, CA 94105
United States of America
Email: szandi@linkedin.com
Wim Henderickx
Nokia
copernicuslaan 50
2018 Antwerp
Belgium
Email: wim.henderickx@nokia.com
Patel, et al. Expires 14 September 2023 [Page 38]
