INTERNET DRAFT IS-IS Multi-instance Multi-topology Jun 2006
Network Working Group S. Previdi
Internet Draft L. Ginsberg
Expiration Date: Dec 2006 M. Shand
A. Roy
D. Ward
Cisco Systems
June 2006
IS-IS Multi-instance Multi-topology
draft-previdi-isis-mi-mt-01.txt
Status of this Memo
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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
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an alternative to Multi-Topology IS-IS. Routers will form instance
specific adjacencies, exchange instance specific routing updates and
compute paths utilizing instance specific LSDB information. 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.
Table of Contents
1. Conventions used in this document..............................2
2. Introduction...................................................2
3. Proposed Solution..............................................3
3.1 Instance Identifier..........................................3
3.2 Instance Membership..........................................3
3.3 Adjacency Establishment......................................4
3.3.1 Point-to-Point Adjacencies................................4
3.3.2 Multi-Access Adjacencies..................................4
3.4 Interoperability Considerations..............................4
3.4.1 MI-ISIS Layer 2 multicast address.........................5
3.4.2 Interoperability using p2p networks.......................5
3.4.3 Interoperability using Broadcast networks.................5
4. Security Considerations........................................6
5. IANA Considerations............................................6
6. Normative References...........................................6
7. Acknowledgments................................................6
8. Authors' Addresses.............................................7
9. Full Copyright Statement.......................................7
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 [BCP14].
2. Introduction
"[MT-IS-IS] defines extensions to IS-IS which support multiple
topologies through the use of additional TLVs in IIH/LSP PDUs. [MT-
IS-IS] specifies that a single adjacency, single flooding scheme,
and single LSDB are to be shared across all topologies to which a
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router belongs. This draft describes an alternative approach where
multiple topologies are supported by the use of multiple independent
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 IS-IS 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,
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
A new TLV is defined in order to convey an instance identifier
(IID). The purpose 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. Multiple
instances of IS-IS may co-exist on the same network and on the same
physical router. IIDs MUST be unique within the same routing domain.
Instance identifier #0 is reserved for the standard topology
supported by legacy systems.
The following format is used for the IID:
Type TBA by IANA
Length 2
Value <16-bit number IID>
3.2 Instance Membership
Each router is configured to be participating in one or more
instances of IS-IS. For each instance in which it participates, a
router labels all IS-IS PDUs (IIH, LSP or SNP) generated pertaining
to that instance by including the appropriate IID TLV. Note that
this applies for the standard topology (instance identifier #0). A
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PDU can only be labeled with a single instance identifier. 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 when establishing 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.
This extension allows an MI-RTR to establish multiple adjacencies to
the same physical neighbor over a p2p link. This differs from the
normal behavior on 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
that PDUs sent by one router are visible to all routers and all
routers must agree on the election of a DIS.
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.
3.4 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 unless the mechanisms and procedures
discussed below are followed.
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3.4.1 MI-ISIS Layer 2 multicast address
In order for routers to correctly interoperate with routers not
implementing this extension and in order not to cause disruption, a
specific and dedicated MAC address is used for multicasting IS-IS
PDUs labeled with any non-zero IID among MI-RTRs. Each level will
use a specific layer 2 multicast address. Such an address allows MI-
RTRs to exchange IS-IS PDUs with non-zero IIDs without these PDUs
being processed by legacy routers and therefore no disruption is
caused.
An MI-RTR will exchange ISIS PDUs intended for IID #0 using AllL1IS
and AllL2IS ISIS mac layer addresses (as defined in [IS-IS]) and
will use two new (TBD) dedicated layer 2 multicast addresses (one
for each level) when sending IS-IS PDUs for any non-zero IID.
MI-RTRs MUST discard IS-IS PDUs received if either of the following
is true:
. The destination multicast address is AllL1IS or AllL2IS and the
PDU contains an IID TLV with non-zero value.
. The destination multicast address is one of the two new
addresses and the PDU contains an IID TLV with a zero value or
has no IID TLV.
3.4.2 Interoperability using p2p networks
In order for an instance on an MI-RTR which participates in the
standard topology (IID #0) to interoperate over a p2p link with a
router which does NOT support this extension, the MI-RTR MUST NOT
send IS-IS PDUs for instances other than IID #0 over the p2p link as
these PDUs may affect the state of 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.
3.4.3 Interoperability using Broadcast networks
If the multicast addresses AllL1IS and/or AllL2IS are improperly
used to send IS-IS PDUs for non-zero IIDs, legacy systems will
interpret these PDUs as being associated with IID #0. This will
cause inconsistencies in the LSDB in those routers, may incorrectly
maintain adjacencies, and may lead to inconsistent DIS election.
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4. 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.
5. IANA Considerations
This document requires the definition a new ISIS TLV that needs to
be reflected in the ISIS TLV code-point registry:
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
TBA MI-MT IID y y y
6. Normative References
[IS-IS] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002, Second Edition.
[MT-IS-IS] Pryzgienda, T., Shen, N., and Sheth, N., "Multi
Topology (MT) Routing in IS-IS", draft-ietf-isis-wg-multi-
topology-11.txt (work in progress), October 2005.
[HMAC-MD5] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic Authentication", RFC
3567, July 2003.
[BCP9] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[BCP26] Narten, T. and Alvestrand, H., "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26 , RFC 2434, October
1998
[BCP79] Bradner, S. Ed., "Intellectual Property Rights in IETF
Technology ", BCP 79 , RFC 3979, March 2005
7. Acknowledgments
The authors would like to acknowledge contributions made by Dino
Farinacci.
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8. Authors' Addresses
Stefano Previdi
CISCO Systems, Inc.
Via Del Serafico 200
00142 - Roma
ITALY
Email: sprevidi@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
Abhay Roy
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
akr@cisco.com
Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
Email: mshand@cisco.com
Dave Ward
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
dward@cisco.com
9. Full Copyright Statement
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