Internet DRAFT - draft-ietf-l3vpn-rt-constrain
draft-ietf-l3vpn-rt-constrain
L3VPN Working Group P. Marques
Internet-Draft R. Bonica
Expires: December 24, 2005 Juniper Networks
L. Fang
AT&T
L. Martini
R. Raszuk
K. Patel
J. Guichard
Cisco Systems, Inc.
June 22, 2005
Constrained VPN Route Distribution
draft-ietf-l3vpn-rt-constrain-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines MP-BGP procedures that allow BGP speakers to
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exchange Route Target reachability information. This information can
be used to build a route distribution graph in order to limit the
propagation of VPN NLRI (such as VPN-IPv4, VPN-IPv6 or L2-VPN NLRI)
between different autonomous systems or distinct clusters of the same
autonomous system.
Table of Contents
1. Specification of Requirements . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. NLRI Distribution . . . . . . . . . . . . . . . . . . . . . 6
3.1 Inter-AS VPN Route Distribution . . . . . . . . . . . . . 6
3.2 Intra-AS VPN Route Distribution . . . . . . . . . . . . . 7
4. Route Target membership NLRI advertisements . . . . . . . . 10
5. Capability Advertisement . . . . . . . . . . . . . . . . . . 11
6. Operation . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Deployment Considerations . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . 14
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1 Normative References . . . . . . . . . . . . . . . . . . 16
10.2 Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 19
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1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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2. Introduction
In BGP/MPLS IP VPNs, PE routers use Route Target (RT) extended
communities to control the distribution of routes into VRFs. Within
a given iBGP mesh, PE routers need only to hold routes marked with
Route Targets pertaining to VRFs that have local CE attachments.
It is common, however, for an autonomous system to use route
reflection [2] in order to simplify the process of bringing up a new
PE router in the network and to limit the size of the iBGP peering
mesh.
In such a scenario, as well as when VPNs may have members in more
than one autonomous system, the number of routes carried by the
inter-cluster or inter-as distribution routers is an important
consideration.
In order to limit the VPN routing information that is maintained at a
given route reflector, RFC2547bis [3] suggests, in section 4.3.3.,
the use of "Cooperative Route Filtering" [4] between route
reflectors. This proposal extends the RFC2547bis [3] ORF work to
include support for multiple autonomous systems, and asymmetric VPN
topologies such as hub-and-spoke.
While it would be possible to extend the encoding currently defined
for the extended-community ORF in order to achieve this purpose, BGP
itself already has all the necessary machinery for dissemination of
arbitrary information in a loop free fashion, both within a single
autonomous system, as well as across multiple autonomous systems.
This document builds on the model described in RFC2547bis [3] and on
concept of cooperative route filtering by adding the ability to
propagate Route Target membership information between iBGP meshes.
It is designed to supersede "cooperative route filtering" for VPN
related applications.
By using MP-BGP UPDATE messages to propagate Route Target membership
information it is possible to reuse all this machinery including
route reflection, confederations and inter-as information loop
detection.
Received Route Target membership information can then be used to
restrict advertisement of VPN NLRI to peers that have advertised
their respective Route Targets, effectively building a route
distribution graph. In this model, VPN NLRI routing information
flows in the inverse direction of Route Target membership
information.
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This mechanism is applicable to any BGP NLRI that controls the
distribution of routing information based on Route Targets, such as
BGP L2VPNs [?] and VPLS [9].
Throughout this document, the term NLRI, which originally expands to
"Network Layer Reachability Information" is used to describe routing
information carried via MP-BGP updates without any assumption of
semantics.
An NLRI consisting of {origin-as#, route-target} will be referred to
as RT membership information for the purpose of the explanation in
this document.
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3. NLRI Distribution
3.1 Inter-AS VPN Route Distribution
In order to better understand the problem at hand, it is helpful to
divide it in its inter-AS and intra-AS components. Figure 1
represents an arbitrary graph of autonomous systems (a through j)
interconnected in an ad-hoc fashion. The following discussion
ignores the complexity of intra-AS route distribution.
+----------------------------------+
| +---+ +---+ +---+ |
| | a | -- | b | -- | c | |
| +---+ +---+ +---+ |
| | | |
| | | |
| +---+ +---+ +---+ +---+ |
| | d | -- | e | -- | f | -- | j | |
| +---+ +---+ +---+ +---+ |
| / | |
| / | |
| +---+ +---+ +---+ |
| | g | -- | h | -- | i | |
| +---+ +---+ +---+ |
+----------------------------------+
Figure 1: Topology of autonomous systems
Lets consider the simple case of a VPN with CE attachments in ASes a
and i using a single Route Target to control VPN route distribution.
Ideally we would like to build a flooding graph for the respective
VPN routes that would not include nodes (c, g, h, j). Nodes (c, j)
are leafs ASes that do not require this information while nodes (g,
h) are not in the shortest inter-as path between (e) and (i) and thus
should be excluded via standard BGP path selection.
In order to achieve this we will rely on ASa and ASi generating a
NLRI consisting of {origin-as#, route-target} ( RT membership
information ). Receipt of such an advertisement by one of the ASes
in the network will signal the need to distribute VPN routes
containing this Route Target community to the peer that advertised
this route.
Using RT membership information that includes both route-target and
originator AS number, allows BGP speakers to use standard path
selection rules concerning as-path length (and other policy
mechanisms) to prune duplicate paths in the RT membership information
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flooding graph, while maintaining the information required to reach
all autonomous systems advertising the Route Target.
In the example above, AS e needs to maintain a path to AS a in order
to flood VPN routing information originating from AS i and vice-
versa. It should however, as default policy, prune less preferred
paths such as the longer path to ASi with as-path (g h i).
Extending the example above to include AS j as a member of the VPN
distribution graph would cause AS f to advertise 2 RT Membership NLRI
to AS e, one containing origin AS i and one containing origin AS j.
While advertising a single path would be sufficient to guarantee that
VPN information flows to all VPN member ASes, this is not enough for
the desired path selection choices. In the example above, assume (f
j) is selected and advertised. Where that to be the case the
information concerning the path (f i), which is necessary to prune
the arc (e g h i) from the route distribution graph, would be
missing.
As with other approaches for building distribution graphs, the
benefits of this mechanism are directly proportional to how "sparse"
the VPN membership is. Standard RFC2547 inter-AS behavior can be
seen as a dense-mode approach, to make the analogy with multicast
routing protocols.
3.2 Intra-AS VPN Route Distribution
As indicated above, the inter-AS VPN route distribution graph, for a
given route-target, is constructed by creating a directed arc on the
inverse direction of received Route Target membership UPDATEs
containing an NLRI of the form {origin-as#, route-target}.
Inside the BGP topology of a given autonomous-system, as far as
external RT membership information is concerned (route-targets where
the as# is not the local as), it is easy to see that standard BGP
route selection and advertisement rules [5] will allow a transit AS
to create the necessary flooding state.
Consider a IPv4 NLRI prefix, sourced by a single AS, which is
distributed via BGP within a given transit AS. BGP protocol rules
guarantee that a BGP speaker has a valid route that can be used for
forwarding of data packets for that destination prefix, in the
inverse path of received routing updates.
By the same token, and given that a {origin-as#, route-target} key
provides uniqueness between several ASes that may be sourcing this
route-target, BGP route selection and advertisement procedures
guarantee that a valid VPN route distribution path exists to the
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origin of the Route Target membership information advertisement.
Route Target membership information that is originated within the
autonomous-system however requires more careful examination. Several
PE routers within a given autonomous-system may source the same NLRI
{origin-as#, route-target}, thus default route advertisement rules
are no longer sufficient to guarantee that within the given AS each
node in the distribution graph has selected a feasible path to each
of the PEs that import the given route-target.
When processing RT membership NLRIs received from internal iBGP
peers, it is necessary to consider all available iBGP paths for a
given RT prefix, when building the outbound route filter, and not
just the best path.
In addition, when advertising Route Target membership information
sourced by the local autonomous system to an iBGP peer, a BGP speaker
shall modify its procedure to calculate the BGP attributes such that:
i. When advertising RT membership NLRI to a route-reflector
client, the Originator attribute shall be set to the router-id of
the advertiser and the Next-hop attribute shall be set of the
local address for that session.
ii. When advertising a RT membership NLRI to a non client peer,
if the best path as selected by path selection procedure described
in section 9.1 of the base BGP specification [5] is a route
received from a non-client peer, and there is an alternative path
to the same destination from a client, the attributes of the
client path are advertised to the peer.
The first of these route advertisement rules is designed such that
the originator of RT membership NLRI does not drop a RT membership
NLRI which is reflected back to it, thus allowing the route reflector
to use this RT membership NLRI in order to signal the client that it
should distribute VPN routes with the specific target torwards the
reflector.
The second rule makes it such that any BGP speaker present in an iBGP
mesh can signal the interest of its route reflection clients in
receiving VPN routes for that target.
These procedures assume that the autonomous-system route reflection
topology is configured such that IPv4 unicast routing would work
correctly. For instance, route reflection clusters must be
contiguous.
An alternative solution to the procedure given above would have been
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to source different routes per PE, such as NLRI of the form {origina-
tor-id, route-target}, and aggregate them at the edge of the network.
The solution adopted is considered to be advantageous over the former
given that it requires less routing-information within a given AS.
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4. Route Target membership NLRI advertisements
Route Target membership NLRI is advertised in BGP UPDATE messages
using the MP_REACH_NLRI and MP_UNREACH_NLRI attributes [6]. The
[AFI, SAFI] value pair used to identify this NLRI is (AFI=1,
SAFI=132).
The Next Hop field of MP_REACH_NLRI attribute shall be interpreted as
an IPv4 address, whenever the length of NextHop address is 4 octets,
and as a IPv6 address, whenever the length of the NextHop address is
16 octets.
The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
of 0 to 96 bits encoded as defined in section 4 of [6].
This prefix is structured as follows:
+-------------------------------+
| origin as (4 octets) |
+-------------------------------+
| route target (8 octets) |
+ +
| |
+-------------------------------+
Except for the default route target, which is encoded as a 0 length
prefix, the minimum prefix length is 32 bits. As the origin-as field
cannot be interpreted as a prefix.
Route targets can then be expressed as prefixes, where for instance,
a prefix would encompass all route target extended communities
assigned by a given Global Administrator [7].
The default route target can be used to indicate to a peer the
willingness to receive all VPN route advertisements such as, for
instance, the case of a route reflector speaking to one of its PE
router clients.
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5. Capability Advertisement
A BGP speaker that wishes to exchange Route Target membership
information must use the Multiprotocol Extensions Capability Code as
defined in RFC 2858 [6], to advertise the corresponding (AFI, SAFI)
pair.
A BGP speaker MAY participate in the distribution of Route Target
information while not using the learned information for purposes of
VPN NLRI output route filtering, although the latter is discouraged.
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6. Operation
A VPN NLRI route should be advertised to a peer that participates in
the exchange of Route Target membership information if that peer has
advertised either the default Route Target membership NLRI or a Route
Target membership NLRI containing any of the targets contained in the
extended communities attribute of the VPN route in question.
When a BGP speaker receives a BGP UPDATE that advertises or withdraws
a given Route Target membership NLRI, it should examine the RIB-OUTs
of VPN NLRIs and re-evaluate the advertisement status of routes that
match the Route Target in question.
A BGP speaker should generate the minimum set of BGP VPN route
updates necessary to transition between the previous and current
state of the route distribution graph that is derived from Route
Target membership information.
An an hint that initial RT membership exchange is complete
implementations SHOULD generate an End-of-RIB marker, as defined in
[8], for the Route Target membership (afi, safi). Regardless of
whether graceful-restart is enabled on the BGP session. This allows
the receiver to know when it has received the full contents of the
peers membership information. The exchange of VPN NLRI should follow
the receipt of the End-of-RIB markers.
If a BGP speaker chooses to delay the advertisement of BGP VPN route
updates until it receives this End-of-RIB marker, it MUST limit that
delay to an upper bound. By default, a 60 second value should be
used.
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7. Deployment Considerations
This mechanism reduces the scaling requirements that are imposed on
route reflectors by limiting the number of VPN routes and events that
a reflector has to process to the VPN routes used by its direct
clients. By default, a reflector must scale in terms of the total
number of VPN routes present on the network.
This also means that its is now possible to reduce the load imposed
on a given reflector by dividing the PE routers present on its
cluster into a new set of clusters. This is a localized
configuration change that need not affect any system outside this
cluster.
The effectiveness of RT-based filtering depends on how sparse the VPN
membership is.
The same policy mechanisms applicable to other NLRIs are also
applicable to RT membership information. This gives a network
operator the option of controlling which VPN routes get advertised in
an inter-domain border by filtering the acceptable RT membership
advertisements inbound.
For instance, in the inter-as case, it is likely that a given VPN is
connected to only a subset of all participating ASes. The only
current mechanism to limit the scope of VPN route flooding is through
manual filtering on the EBGP border routers. With the current
proposal such filtering can be performed based on the dynamic Route
Target membership information.
In some inter-as deployments not all RTs used for a given VPN have
external significance. For example, a VPN can use an hub RT and a
spoke RT internally to an autonomous-system. The spoke RT does not
have meaning outside this AS and so it may be stripped at an external
border router. The same policy rules that result in extended
community filtering can be applied to RT membership information in
order to avoid advertising an RT membership NLRI for the spoke-RT in
the example above.
Throughout this document, we assume that autonomous-systems agree on
an RT assignment convention. RT translation at the external border
router boundary, is considered to be a local implementation decision,
as it should not affect inter-operability.
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8. Security Considerations
This document does not alter the security properties of BGP-based
VPNs. However it should be taken into consideration that output
route filters built from RT membership information NLRI are not
intended for security purposes. When exchanging routing information
between separate administrative domains, it is a good practice to
filter all incoming and outgoing NLRIs by some other means in
addition to RT membership information. Implementations SHOULD also
provide means to filter RT membership information.
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9. Acknowledgments
This proposal is based on the extended community route filtering
mechanism defined in [4].
Ahmed Guetari was instrumental in defining requirements for this
proposal.
The authors would also like to thank Yakov Rekhter, Dan Tappan, Dave
Ward, John Scudder, and Jerry Ash for their comments and suggestions.
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10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An
Alternative to Full Mesh IBGP", RFC 2796, April 2000.
[3] Rosen, E., "BGP/MPLS IP VPNs", draft-ietf-l3vpn-rfc2547bis-03
(work in progress), October 2004.
[4] Chen, E. and Y. Rekhter, "Cooperative Route Filtering Capability
for BGP-4", draft-ietf-idr-route-filter-11 (work in progress),
December 2004.
[5] Rekhter, Y., "A Border Gateway Protocol 4 (BGP-4)",
draft-ietf-idr-bgp4-26 (work in progress), October 2004.
[6] Bates, T., Rekhter, Y., Chandra, R., and D. Katz, "Multiprotocol
Extensions for BGP-4", RFC 2858, June 2000.
[7] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", draft-ietf-idr-bgp-ext-communities-08
(work in progress), February 2005.
[8] Sangli, S., Rekhter, Y., Fernando, R., Scudder, J., and E. Chen,
"Graceful Restart Mechanism for BGP", draft-ietf-idr-restart-10
(work in progress), June 2004.
10.2 Informative References
[9] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service",
draft-ietf-l2vpn-vpls-bgp-05 (work in progress), April 2005.
Authors' Addresses
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: roque@juniper.net
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Ronald Bonica
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: rbonica@juniper.net
Luyuan Fang
AT&T
200 Laurel Avenue, Room C2-3B35
Middletown, NJ 07748
US
Email: luyuanfang@att.com
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
US
Email: lmartini@cisco.com
Robert Raszuk
Cisco Systems, Inc.
170 West Tasman Dr
San Jose, CA 95134
US
Email: rraszuk@cisco.com
Keyur Patel
Cisco Systems, Inc.
170 West Tasman Dr
San Jose, CA 95134
US
Email: keyupate@cisco.com
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Jim Guichard
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
US
Email: jguichar@cisco.com
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