Working Group Peter Ashwood-Smith
Internet-Draft Don Fedyk
Date Created: June, 2009 David Allan
Expiration Date: January, 2010 Jerome Chiabaut
Intended Status: Informational Nigel Bragg
Preliminary
Shortest Path Bridging and Backbone Bridging with IS-IS
draft-fedyk-isis-spb-00.txt
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Fedyk et al. [Page 1]
Internet Draft draft-fedyk-isis-spb-00.txt July 2009
Abstract
Several techniques are being developed which use IS-IS to deliver
link state based layer 2 forwarding. The superset of the extensions
proposed to IS-IS to allow these capabilities is found in [IS-IS-
L2]. One technique for layer 2 forwarding is being specified in
the IEEE 802.1aq task group, under the over-arching title
of "Shortest Path Bridging" (SPB). SPB however only requires a
subset of the proposed IS-IS extensions in [IS-IS-L2]. For clarity
this informational draft documents only the subset required by SPB.
In addition a high level introduction, describing how these TLVs
are used is provided for those who do not follow the IEEE work in
detail. A reference is also given to the normative IEEE 802.1aq
document The ordering of material in this document follows that of
Clause 28 of IEEE 802.1aq, to aid cross-referencing.
Conventions used in this document
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 [RFC2119].
Table of Contents
1. Terminology..................................................3
2. Introduction.................................................4
3. New TLVs for SPB and SPBB....................................6
3.1 Base Vlan-Identifiers sub-TLV................................7
3.2 SPB/SPBB Instance sub-TLV....................................8
3.3 SPB Link Metric sub-TLV......................................9
3.4 The Group MAC Address sub-TLV................................9
3.5 The Service Identifier and Unicast Address sub-TLV..........10
4. Security Considerations.....................................10
5. IANA Considerations / ISO Considerations....................10
6. References..................................................11
6.1 Normative References........................................11
6.2 Informative References......................................11
7. Acknowledgments.............................................12
8. Author's Addresses..........................................12
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1. Terminology
In addition to well understood IS-IS terms, this memo uses
terminology from IEEE 802.1 and introduces a few new terms:
802.1ah Provider Backbone Bridges a.k.a. Mac-in-Mac
encapsulation
802.1aq Shortest Path Bridging (SPB)
B-DA Backbone Destination Address in 802.1ah Mac-in-Mac
header
B-MAC Backbone MAC Address
B-SA Backbone Source address in 802.1ah Mac-in-Mac header
B-VID Backbone VLAN ID in 802.1ah Mac-in-Mac header
B-VLAN Backbone Virtual LAN
C-MAC Customer MAC. Inner MAC in 802.1ah Mac-in-Mac header
C-VID Customer VLAN ID
C-VLAN Customer Virtual LAN
DA Destination Address
FIB Forwarding information base (B-DA/B-VID to next hop(s))
ISID 802.1ah: service membership in datapath(not always=SID)
MAC Media Access Control
PBB Provider Backbone Bridges as specified in 802.1ah
M-IS-IS Multi Topology IS-IS as used in [MT]
MT Multi Topology. As used in [MT]
MT-ID Multi Topology Identifier (12 bits). As used in [MT]
SPSourceID 20 bit nodal identifier used with SID forms mcast DA.
PATHID The unique identifier for a path used for symmetric tie
breaking
PBB Provider Backbone Bridge
PBT Provider Backbone Transport
SA Source Address
SID Service Identifier at control plane, groups many ISIDs
UNI User Network Interface: Customer/Backbone attach point
VID VLAN ID
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2. Introduction
Shortest Path Bridging (SPB) and Shortest Path Backbone Bridging
(SPBB) both apply the [IS-IS] protocol to the utilization of mesh
topologies for native Ethernet bridging. While 802.1aq has the
umbrella title of shortest path bridging, this memo adopts the
convention of ascribing unique attributes to the terms SPB and SPBB
individually; SPB uses an IEEE 802.1Q forwarding paradigm, whilst
SPBB extends the FIB population techniques specified in 802.1Qay
combined with IEEE 802.1ah [PBB] adaptation. Both SPB and SPBB
forward packets on shortest path trees with minimum path cost as a
first order tie-breaker, where for any pair of nodes A and B, the
unicast path for A to B is the exact reverse of the path from B to A
(reverse path congruency), and all multicast traffic between the two
nodes follows the unicast path (multicast and unicast congruency).
These are direct extensions to fundamental Ethernet forwarding
properties in IEEE bridged networks.
In SPB, conventional bridge learning is used to associate (customer)
MAC addresses to ports and hence routes through the SPB region. The
source-rooted tree associated with each node is assigned a unique
VLAN ID (the SPVID) to identify it.
The 802.1ah [PBB] MAC-in-MAC encapsulation used by SPBB permits the
isolation of customer Ethernet addressing from backbone Ethernet
addressing in the core of a network. This has an important
consequence; the association between a customer MAC (C-MAC) and a B-
MAC to resolve forwarding across the core is required only at the
edge of the network. Flooding is only done by the edge adaptation
functions to learn which B-MAC reaches a given C-MAC using the
normal PBB C-MAC learning behavior, flooding at the C-MAC layer
resolving to a service specific multicast tree at the B-MAC layer.
A minimum of one B-VID MUST be assigned to each instance of SPBB
(IS-IS MT-ID). The B-VID MUST be unique backbone network wide. Two
B-VIDs MAY be used by a single SPBB instance (MT-ID) when it is
desired to use more than one equal cost shortest path permutation.
With Spanning Tree Protocol (STP), Rapid STP or Multiple STP[802.1Q]
Ethernet networks use a shared spanning tree (or a small number of
shared trees) to route traffic based on the VLAN ID and then on
learned MAC addresses. Per service multicast is instantiated as a
(*,G) multicast address which is a proper subset of the VID.
In order to move from a shared spanning tree to mesh connectivity,
SPB and SPBB use one or more broadcast trees per source as a
template for per service multicast trees i.e.(S,G).
SPB uses the SPVID when flooding, with conventional pruning as C-MAC
addresses are learned.
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SPBB adapts client flooding onto service specific multicast trees
instantiated by encoding S,G in the destination MAC address. SPBB
therefore uses both B-VID and B-MAC when forwarding.
SPBB uses algorithmically constructed addresses for the multicast
DA. Multicast addresses are local to a PBB domain. The multicast B-
MAC DA address constructed for a particular node/service consists of
a unique nodal identifier called a SPSourceID (20 bits), combined
with the service identifier (SID) (24 bits). Every SPBB node(per
SPBB instance/MT-ID) therefore advertises this unique SPSourceID
which can be correlated to the node's ISIS SYSID by all nodes in the
SPBB network.
In SPB, a VID (called the Base VID) is also defined for
interoperation with xSTP regions. This VID identifies the Common and
Internal Spanning Tree (CIST) in the Shortest Path Region. It is
created as a conventional Spanning Tree within the SPB region, and
Group MAC addresses, installed by conventional Ethernet mechanisms,
may be supported on this VID.
A node performing SPB or SPBB calculations (for a given instance/MT-
ID) use the IS-IS topology, and link metrics to compute which leaves
of each shortest path tree require transit of the local node, and
this node can then do pair-wise comparison of services of interest
between the root and the leaves to populate the FIB accordingly. The
link metrics are forced to be equal in both directions by defaulting
to the largest of the two unidirectional metrics specified for a
link as seen during the establishment of a standard IS-IS adjacency.
A node performing SPB or SPBB for a given MT instance MUST specify
the NLPID for SPB and SPBB (IANA allocation 0xC1 pending) in its IS-
IS hello message. Links that do not support this NLPID for this MT
instance must be excluded from the shortest path computations by the
given MT instance.
Both SPB and SPBB employ a transitive symmetric tie breaking
algorithm which chooses deterministically and progressively between
equal cost alternatives by ranking the paths according to the sorted
list of node identifiers that make up the path. This sorted list of
node identifiers (SYSIDs) is called the Path Identifier (PATHID).
For SPB, and when only one B-VID is used in SPBB, the tie breaking
algorithm will always pick from a set of equal cost shortest path
alternatives by choosing the path with the lowest PATHID. Two B-VID
instances MAY be used for an SPBB instance, when it will assign the
lowest PATHID paths to one B-VID and the highest PATHID paths to the
other B-VID. In this manner an SPBB instance will use more of the
available paths in a network but will do so by assignment of packets
at the head ends to one of different SPBB B-VIDs.
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3. New TLVs for SPB and SPBB
SPB and SPBB require a subset of the [MT] TLVs.
SPB and SPBB inherit the Multi Topology mechanisms from [MT] to
allow multiple logical bridging instances to exist within a single
IS-IS control instance. Top level TLV's used by SPB and SPBB
therefore begin with the Multi Topology Identifier (MT-ID) fields as
defined in [MT] and the new sub-TLVs identified here may be used as
sub-TLVs of the corresponding new top level Multi Topology TLV's
defined in [MT].
In the IEEE the Multiple Spanning tree protocol allowed multiple
VIDs to represent a single VLAN. The single "logical" VLAN was
identified by a Base VID so that bridges external to the region
could have a consistent VID to identify the VLAN. This concept of a
Base VID extends easily to SPB and SPBB. In this way a Base VID can
be used to identify a topology instance and to correlate VIDs that
are allocated to a particular shortest path VLAN instance.
A Shortest Path Region also has the property that viewed from
outside the Region it appears as a single 802.1Q or 802.1ah bridge,
irrespective of how the VLAN is implemented.
In SPB, each node uses a unique VID as its source identifier (an
SPVID), and each SPVID is correlated to a Base VID. The set of
SPVIDs that map to a given Base VID form the SPB region. In SPBB,
this concept is reused however there is a slight modification. A
single B-VID maps to a Base VID for all bridges. However multiple B-
VIDs (currently two) can map to the same Base VID allowing multiple
trees within the SPBB VLAN. A typical use for multiple trees is to
instantiate equal cost paths and provide the opportunity to load
spread services.
A Base VID identifies a traditional spanning tree in both SPB and
SPBB that can be used to represent the VLAN for proper bridge
behavior when viewed by bridges outside the shortest path region.
In essence the VLAN identified by this Base VID can appear as a
single bridge allowing proper spanning tree behavior.
The following sections introduce the new [MT] TLVs which are used by
SPB and SPBB, and give an overview of their use from an SPB and SPBB
perspective.
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3.1 Base Vlan-Identifiers sub-TLV
The Base Vlan-Identifier sub-TLV (section 2.3.5 of [IS-IS-L2]) is
carried within the Multi Topology aware Port Capability TLV (section
2.3 of [IS-IS-L2]), and this is carried in an IIH PDU.
The purpose of this sub-TLV is to check for compatible configuration
of SPB or SPBB mode of operation and then of SPB or SPBB parameters
as bridges form adjacencies, and to prevent adjacency formation when
incompatible configurations are detected.
In informal terms this requires :
- agreement on the NLPID for SPB and SPBB
- specification of the bridging mode to be used (SPB or SPBB)
- binding of individual VIDs to the Base VID, and specification of
the shortest path bridging algorithm to be used for each VID
- use (or not) of auto allocation capability for SPVIDs in SPB and
SPSourceIDs in SPBB.
It was mentioned earlier that each SPB node is assigned a unique VID
(a Shortest Path VID, or SPVID) as its source identifier for all
traffic it transmits. The set of SPVIDs, one for each SPB node in
the region, are bound to the Base VID, configured to execute a
specific tiebreaking algorithm, and collectively provide the
shortest path trees to support the VLAN. A control flag in the sub-
TLV determines whether the SPVIDs are provisioned, or auto-allocated
by the procedure in [801.1aq]
In SPB, the Base VID is also used to identify a VLAN providing peer
inter-working with other non-SPB bridges outside the SPB Region.
This VLAN forms a spanning tree across the region to achieve this.
In SPBB, the VID(s) on which forwarding is performed are Region-wide
assignments. At present, the use of one or two VIDs is defined,
with the latter capability available for edge-based load spreading
using Equal Cost Multiple Trees generated via the symmetric tie-
breaking variations.
Three algorithms are currently available to SPB and SPBB :
- spanning tree algorithm, which constructs a tree which is the
same as would be constructed by Spanning Tree Protocol [802.1D]
- shortest path trees, selecting the low PATHID as a tie-breaker
- shortest path trees, selecting the high PATHID as a tie-breaker
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3.2 SPB/SPBB Instance sub-TLV
The SPB/SPBB Instance sub-TLV (section 2.5.1 of [IS-IS-L2]) is
carried within the Multi Topology Aware Capability TLV (section 2.5
of [IS-IS-L2]).
The purpose of this sub-TLV is to announce configuration and other
parameters to the entire SPB or SPBB Region. The instance sub-TLV
carries some information elements common to the Base VLAN-
Identifiers sub-TLV described in the previous section, which are :
- binding of individual VIDs to the Base VID, and specification of
the shortest path bridging algorithm to be used for each VID
- use (or not) of auto allocation capability for SPVIDs in SPB and
SPSourceIDs in SPBB.
The SPB/SPBB Instance sub-TLV carries further information :
- the SPSourceID - the 20 bit network wide unique identifier used
in the higher order bits of the SPBB multicast DA for packets
originating at this node
- various Spanning Tree parameters for inter-working with non-SPB
regions
In SPB, the multicast tree built off each SPB node is uniquely
associated with an SPVID which thereby identifies the source. The
required (S,G) trees, and loop avoidance checking, may be directly
implemented using this SPVID by standard Ethernet forwarding. The
SPVID can be configured from a pool or it can be auto allocated.
In SPBB multicast, the same capability is achieved by encoding the
service-specific (S,G) tree in the multicast Destination Address.
This is achieved by concatenating the PBB Service Identifier with
the nodal SPSourceID. The distribution of SPSourceID therefore
allows all SPBB nodes to compute the forwarding state they need to
install, based only on topology and service endpoint locations. The
computed SPBB multicast DA looks like this:
+-+-+-+-+-----------------------+---------------------------+
|M/L| A | SPSourceID (20 bit) | I-SID (24 bit) |
+-+-+-+-+-----------------------+---------------------------+
Where M/L = multicast/local bits
(2 bits - both set to 1)
A = SPSourceID allocation style
(2 bits - both 0 initially)
Figure 1: SPBB multicast MAC address construction
The SPSourceID can be provisioned, or auto allocated.
The Spanning tree inter-working parameters for SPB comprise :
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- a Spanning tree compatible Bridge identifier, configured exactly
as specified in [802.1D]. This allows SPB to build a compatible
Spanning tree using link state.
- The Base VID identifies a VLAN capable of covering multiple
Regions, SPB and non-SPB. In SPB, this is known as the Common
and Internal Spanning Tree (CIST). At SPB Region boundaries, the
CIST Root Identifier and the CIST External Root Path Cost may be
imported from xSTP and flooded by IS-IS as a part of SPB`s
Spanning Tree emulation.
3.3 SPB Link Metric sub-TLV
SPB Link Metric sub-TLV (section 2.6 of [IS-IS-L2]) is carried
within the Extended Reachability TLV, or the Multi Topology
Intermediate System TLV.
The purpose of this sub-TLV is to announce SPB link metrics, in a
form which enables SPB to build xSTP compatible Spanning Trees,
typically to create the SPB component of the CIST (above) :
- indicates the administrative cost or weight of using a link
- a standard IEEE port identifier used to build a spanning tree
associated with this link
3.4 The Group MAC Address sub-TLV
The Group MAC Address sub-TLV (section 2.2.1 of [IS-IS-L2]) is
carried within the Group Address TLV (section 2.2 of [IS-IS-L2]),
which is in turn carried within the Multicast Group Level 1 link
state PDU.
This sub-TLV is used only by SPB. SPBB builds and installs per-
service per-source multicast addresses algorithmically, as
described earlier, using the SPSourceID and PBB Service Identifier
information announced in other sub-TLVs.
By default, SPB nodes broadcast traffic to all other nodes in their
region. When inter-working with non-SPB regions over the CIST,
multicast group membership may be signaled over the CIST using
mechanisms such as MMRP [802.1ak]. The Group MAC Address sub-TLV
allows such registrations to be imported and announced by IS-IS.
The sub-TLV carries the following information for SPB :
- the VID with which all subsequent MAC addresses are associated
- sets of group records, each consisting of a multicast group
address and a list of unicast MAC addresses known to be sources
of that group.
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3.5 The Service Identifier and Unicast Address sub-TLV
The Service Identifier and Unicast Address sub-TLV (section 2.5.2 of
[IS-IS-L2]) is carried within the Multi Topology Aware Capability
TLV (section 2.5 of [IS-IS-L2]).
The purpose of this sub-TLV is to announce service group membership
(using the PBB Service Identifier or I-SID) on the originating node,
also to advertise an additional B-MAC unicast address present on or
reachable by the node. It is applicable to SPBB only.
This sub-TLV carries:
- the unicast B-MAC address which must be used to send to the set
of PBB I-SIDs announced in the sub-TLV, and which this node will
use as its source B-MAC when transmitting these I-SIDs
- the unicast VID which must be used to send to the set of PBB I-
SIDs announced in the sub-TLV, and which this node will use when
transmitting these I-SIDs
- a list of PBB I-SIDs and their transmit and receive properties.
Announcement of I-SIDs in this way allows all SPBB nodes to see all
service endpoints, and allows nodes not terminating a particular
service to algorithmically determine the per-service per-source
forwarding state which they must install if they lie on the shortest
path between two or more service end-points.
The advertisement of the B-MAC unicast address to be used to reach
the set of services allows different granularities of addressing to
be used within the SPBB node, without compromising inter-working
between nodes of different types. It also has application in some
resiliency schemes.
4. Security Considerations
This document adds no additional security risks to IS-IS.
SPBB assumes that the link state bridged subnetwork consists of
trusted devices and that the UNI ports to the domain are untrusted.
Care is required to ensure untrusted access to the trusted domain
does not occur.
5. IANA Considerations / ISO Considerations
See the subset of [IS-IS-L2] cited by this document and also the
NLPID assignments requested by [NLPID].
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6. References
6.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MT] Multi Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs),
RFC 5120, February 2008
[802.1D] "IEEE draft Standard for Local and Metropolitan
Networks, Media Access Control (MAC) Bridges",
IEEE 802.D June 2004
[802.1aq] "IEEE draft Standard for Local and Metropolitan
Networks, Virtual Bridged Local Area Networks,
Amendment 9: Shortest Path Bridging",
IEEE 802.1aq D2.0, June 2009
[802.1ak] "IEEE Standard for Local and Metropolitan Networks,
Virtual Bridged Local Area Networks, Amendment 7:
Multiple Registration Protocol"
IEEE Std 802.1ak - 2007 amendment to IEEE 802.Q - 2005
[PBB] "IEEE Standard for Local and Metropolitan
Networks, Virtual Bridged Local Area Networks,
Amendment 7: Provider Backbone Bridges"
IEEE Std 802.1ah - 2008 amendment to IEEE 802.Q - 2005
[IS-IS-L2] Extensions to IS-IS for Layer-2 Systems, IETF,
Internet Draft, draft-ietf-isis-layer2-00.txt,
Work in Progress, June 2009
[NLPID] IANA Considerations for NLPIDs, IETF,
Internet Draft, Draft-eastlake-nlpid-iana-
considerations-00.txt,
Work in Progress, June 23, 2009
6.2 Informative References
[IS-IS] ISO/IEC 10589:2002, "Intermediate system to
Intermediate system routing information exchange
protocol" ISO/IEC
10589:2002.
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7. Acknowledgments
The authors would like to thank Antonela Paraschiv, Daniel Joyal,
Paul Unbehagen and Gautam Khera for their detailed review of this
work.
8. Author's Addresses
Don Fedyk
Alcatel-Lucent
220 Hayden Road
Groton, MA, USA
Email donald.fedyk@alcatel-lucent.com
Peter Ashwood-Smith
Huawei Technologies Canada
411 Leggget Drive, Suite 503
Kanata, Ontario, K2k3C9, Canada
Email: peter.ashwoodsmith@huawei.com
Nigel Bragg
Nortel Networks
London Road, Harlow,
Essex CM17 9NA, UK
Email: nbragg@nortel.com
David Allan
Nortel Networks
3500 Carling Ave.
Ottawa, ON, Canada
K1Y4H7
Email: dallan@nortel.com
Jerome Chiabaut
Nortel Networks
3500 Carling Ave.
Ottawa, ON, Canada
K1Y 4H7
Email: chiabaut@nortel.com
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