Network Working Group S. Previdi
Internet Draft D. Ward
L. Ginsberg
Expires: February, 2006 A. Roy
Cisco Systems, Inc
August, 2005
IS-IS Multi-instance Multi-topology
draft-previdi-isis-mi-mt-00.txt
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Internet Draft Multi-Instance IS-IS August, 2005
Abstract
This draft describes a mechanism that allows a single router to
share one or more links among multiple IS-IS routing protocol
instances.
Multiple instances allow the deployment of multiple address-families
as well as multiple instances of the same address-family and it is
an alternative to Multi-Topology IS-IS. Routers supporting the same
instance will form adjacencies, exchange routing updates and compute
paths. Each PDU will contain a new TLV identifying the instance to
which the PDU belongs. This allows a network operator to deploy
multiple IS-IS topologies in parallel, using the same set of links
when required and still have the capability of computing topology
specific paths. This draft does not address the forwarding paradigm
that needs to be used in order to ensure data PDUs are forwarded
according to the topology to which they belong.
1. 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 RFC-2119
[KEYWORDS].
2. Introduction
IS-IS has been already extended in order to support multiple
topologies [MT-ISIS] through the use of additional TLVs in IIH/LSP
PDUs. MT-ISIS specifies that a single adjacency, single flooding
scheme, and single LSDB are to be shared across all topologies to
which a router belongs. This draft describes an alternative approach
where multiple topologies are supported by the use of multiple
instances of the IS-IS protocol. Routers which support this
extension are referred to as "multi-instance capable routers"
(MI-RTR).
3. Proposed Solution
The solution is based on a new TLV called the Instance Identifier
(IID) that is used to mark each routing PDU originated by the
router. Routers form adjacencies and exchange routing updates only
if their IIDs correspond. Each topology is therefore processed
within a separate instance of the IS-IS protocol.
This also implies an instance specific flooding scheme, instance
specific LSDBs and Instance specific routing calculations. It MAY
also imply instance specific routing and forwarding tables. However,
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Internet Draft Multi-Instance IS-IS August, 2005
this aspect is outside the scope of this specification. When
multiple instances share the same link each instance will have a
separate set of adjacencies. Each IS-IS PDU is associated with
only one IS-IS instance.
How multiple instances are implemented is outside the scope of
this specification.
3.1. Instance Identifier (IID)
A new TLV is defined in order to convey an instance identifier
(IID). The scope of the IID is to mark each IS-IS instance running
on a router with a unique 16-bit number. The IID TLV is carried in
all IS-IS PDUs (IIH, SNP, LSP) originated by the router. Routers
have to exchange and agree on instance numbers so that IIDs can be
understood consistently across adjacencies and flooding domain. The
following format is used for the IID:
TLV:
Type: TBD
Length: 2
Value: <16-bit number IID>
3.2 Instance Membership
Each router can be configured as part of one or more instances of
IS-IS. Each instance the router belongs to will correspond to the
value advertised in the IID TLV of IS-IS PDUs originated by that
instance. Only one IID can be advertised in an IIH, LSP, or SNP
PDU. PDUs with multiple IID TLVs MUST be ignored.
3.3 Adjacency Establishment
In order to establish adjacencies, IS-IS routers exchange IIH PDUs.
Two types of adjacencies exist in IS-IS: point-to-point and
broadcast. The following sub-sections describe the additional rules
an MI-RTR MUST follow in order to establish adjacencies.
3.3.1 Point-to-Point Adjacencies
A new IID TLV is inserted into the p2p hello PDUs originated by an
MI-RTR. Upon reception of an IIH, an MI-RTR inspects the received
IID TLV and if it matches any of the IIDs configured on that link,
normal adjacency establishment procedures are used to establish an
instance specific adjacency.
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Internet Draft Multi-Instance IS-IS August, 2005
This extension allows an MI-RTR to establish multiple adjacencies to
the same neighbor over a p2p link. This differs from the generic
behavior of p2p links where only one adjacency is formed. However,
in this case IS-IS instances are "ships-in-the-night" and from a
logical perspective only one adjacency per instance is formed on
p2p links.
3.3.2 Multi-Access Adjacencies
Multi-Access (broadcast) networks behave differently than p2p in the
sense that a DIS is elected. MI-RTRs will establish adjacencies and
elect a DIS per IS-IS instance. Upon reception of an IIH each MI-RTR
will form adjacencies only with routers advertising the same IID in
their IIH PDUs. Since an MI-RTR is not required to participate in
all IIDs on a LAN, it's possible to elect a different DIS for
different instances.
3.3.3 Interoperability Considerations
It is assumed that any TLV that is not understood is silently
ignored without compromising the processing of the whole IS-IS PDU
(IIH, LSP, SNP).
To a router not implementing this extension, all IS-IS PDUs received
will appear to be associated with the standard topology regardless
of any IID TLVs which may be contained in those PDUs. This can cause
interoperability issues, not all of which can be resolved. Therefore
deployment/configuration of MI-RTRs must be done prudently. MI-RTRs
may be configured to accept or not to form an adjacency with a
router not supporting this extension. In any case, only the IID zero
instance can seamlessly interoperate with routers not supporting
this extension.
3.3.3.1 Interoperability using p2p networks
MI-RTRs supporting IID #0 may interoperate over a p2p link with a
router which does NOT support this extension. To do so, an MI-RTR
must refrain from sending LSPs and SNPs for instances other than
IID #0 over the p2p link. It MUST also refrain from sending IIHs
for instance IDs other than zero as these IIHs may affect the state
of the adjacency for IID #0 in the neighbor.
The presence/absence of the IID TLV in an IIH indicates that the
neighbor does/does not support this extension. Once it is determined
that the neighbor does not support this extension, an MI-RTR MUST
NOT send PDUs (including IIHs) for instances other than IID #0.
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Internet Draft Multi-Instance IS-IS August, 2005
Until such time as the capability of the neighbor are known, an
implementation MAY send IIHs for any IID on a p2p link.
3.3.3.2 Interoperability using Multi-Access networks
The presence on a multi-access network of one or more MI-RTRs
supporting one or more non-zero IIDs is incompatible with the
presence of any routers which do not support this extension. This is
because the necessary transmission of IS-IS PDUs associated with
non-zero IIDs will be interpreted as being associated with IID #0 by
the routers not supporting this extension. Therefore, use of this
extension on a multi-access network requires that all routers are
upgraded to a software version supporting this extension. This
restriction MAY be applied independently for each level of routing
supported on the network.
4. IANA considerations
IANA will assign a new codepoint for the MI-MT IID defined in this
document and carried within the IIH PDU. Suggest value is XX (to be
assigned by IANA).
5. Acknowledgments
The authors would like to thank Mike Shand for his valuable input.
6. Normative References
[RFC] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[IS-IS] "Intermediate System to Intermediate System Intra-Domain
Routeing Exchange Protocol for use in Conjunction with the Protocol
for Providing the Connectionless-mode Network Service (ISO 8473)",
ISO 10589.
[IS-IS-IP] Callon, R., RFC 1195, "Use of OSI IS-IS for routing in
TCP/IP and dual environments", RFC 1195, December 1990.
[HMAC-MD5] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic Authentication", RFC 3567,
July 2003.
[MT-IS-IS] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in IS-IS", draft-ietf-isis-wg-multi-topology-
10.txt, May 2005.
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Internet Draft Multi-Instance IS-IS August, 2005
7. Security Considerations
Security concerns for IS-IS are addressed in the IS-IS specification
[IS-IS], and accompanying specifications on [HMAC-MD5]. No
additional considerations need to be made for the extension.
8. Authors' Addresses
Stefano Previdi
Cisco Systems
Via Del Serafico, 200
00142 Rome, Italy
sprevidi@cisco.com
Dave Ward
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
dward@cisco.com
Les Ginsberg
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
ginsberg@cisco.com
Abhay Roy
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
akr@cisco.com
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Internet Draft Multi-Instance IS-IS August, 2005
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This document expires in February, 2006.
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