Network Working Group Amir Hermelin
Internet Draft Charlotte's Web Networks
Expiration Date: September 2002
Stefano Previdi
Mike Shand
Cisco Systems
Extending the Number of IS-IS LSP Fragments Beyond the 256 Limit
draft-ietf-isis-ext-lsp-frags-00.txt
Status
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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].
Abstract
This document describes a mechanism that allows a system to originate
more than 256 LSP fragments, a limit set by the original Intermediate
System to Intermediate System (IS-IS) Routing protocol, as described
in ISO 10589. This mechanism can be used in IP-only, OSI-only, and
dual routers.
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1. Introduction
In the IS-IS protocol, a system floods its link-state information in
Link State Protocol Data Units, or LSPs for short. These logical
LSPs can become quite large, therefore the protocol specifies a means
of fragmenting this information into multiple LSP fragments. The
number of fragments a system can generate is limited by ISO 10589
[ISIS-ISO] to 256 fragments, where each fragment's size is also
limited. Hence, there is a limit on the amount of link-state
information a system can generate.
A number of factors can contribute to exceeding this limit:
- Introduction of new TLVs and sub-TLVs to be included in LSPs.
- The use of LSPs to propagate various types of information (such
as traffic-engineering information).
- The growing size of route tables and AS topologies.
- Finer granularity routing, and the ability to inject external
routes into areas [DOMAIN-WIDE].
- Other emerging technologies, such as optical, IPv6, etc.
This document describes mechanisms to relax the limit on the number
of LSP fragments.
1.1 Definitions of Commonly Used Terms
This section provides definitions for terms that are used throughout
the text.
Originating System
A router running the IS-IS protocol.
Original system-id
The system-id of an Originating System.
Extended system-id
An additional system-id that is assigned by the network
administrator. Each extended system-id allows generation of 256
additional, or extended, LSP fragments. The extended system-id,
like the original system-id, must be unique throughout the
routing domain.
Virtual System
The system, identified by an extended system-id, advertised as
originating the extended LSP fragments. These fragments specify
the extended system-id in their LSP IDs.
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Original LSP
An LSP using the original system-id in its LSP ID.
Extended LSP
An LSP using an extended system-id in its LSP ID.
LSP set
Logical LSP. This term is used only to resolve the ambiguity
between a logical LSP and an LSP fragment, both of which are
sometimes termed "LSP".
Extended LSP set
A group of LSP fragments using an extended system-id, and
originated by the Originating System.
Extension-capable IS
An IS implementing this extension.
1.2 Operation Modes
Two administrative operation modes are provided:
- Operation Mode 1 provides behavior that allows implementations
that don't support this extension, to correctly process the
extended fragment information, without any modifications. This
mode has some restrictions on what may be advertised in the
extended LSP fragments. Namely, only leaf information may be
advertised in the extended LSPs.
- Operation Mode 2 extends the previous mode and relaxes its
advertisement restrictions. Any link-state information may be
advertised in the extended LSPs. However, it mandates a change
to the way LSPs are considered during the SPF algorithm, in a way
that isn't compatible with previous implementations.
These modes may be configured per level. There is no restriction
that an L1/L2 IS operates in the same mode, for both L1 and L2.
Routers MAY implement Operational Mode 2 without supporting running
in Operational Mode 1. They will still interoperate correctly with
routers that support both modes.
1.3 Overview
Using additional system-ids assigned by the administrator, the
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Originating System can advertise the excess link-state information in
extended LSPs under these extended system-ids. It would do so as if
other routers, or "Virtual Systems", were advertising this
information. These extended LSPs will also have the specified limit
on their LSP fragments; however, the Originating System may generate
extended LSPs under numerous Virtual Systems.
For Operation Mode 1, 0-cost adjacencies are advertised from the
Originating System to its Virtual System(s). No adjacencies (other
than back to the Originating System) are advertised in the extended
LSPs. As a consequence, the Virtual Systems are 'stub', meaning they
can only be reached through their Originating System. Therefore,
older implementations do not need modifications in order to correctly
process these extended LSPs.
For both modes, each LSP (set) created by a node will contain on its
fragment-0 a new TLV (IS Alias ID TLV) that contains the Original
system-id and PN Number of the (first) LSP created by the router.
Extension-capable ISs can then use this information and store the
original and extended LSPs as one logical LSP.
2.0 IS Alias ID TLV (IS-A)
The proposed IS-A TLV allows extension-capable ISs to recognize all
LSPs of an Originating System, and combine the original and extended
LSPs for the purpose of SPF computation.
The IS Alias ID TLV is type 24, and contains a new data structure,
consisting of:
7 octets of system Id and pseudonode number
1 octet of length of sub-TLVs
0-247 octets of sub-TLVs,
where each sub-TLV consists of a sequence of
1 octet of sub-type
1 octet of length
0-245 octets of value
Without sub-TLVs, this structure consumes 8 octets per LSP set. This
TLV MUST be included in fragment 0 of every LSP set belonging to an
Originating System.
3.0 Generating LSPs
3.1 Both Operation Modes
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Under both modes, the Originating System MUST include information
binding the Original LSP and the Extended ones. It can do this since
it is trivially an extension-capable IS. This is to ensure other
routers correctly process the extra information in their SPF
calculation. This binding is advertised via a new IS Alias ID TLV,
which is advertised in all fragment 0, whether they belong to the
original LSP or to the extended ones.
+---------------------------------------------+
| Originating System |
| system-id = S |
| is-alias-id = S |
| ( extended system-ids = S', S'' ) |
+---------------------------------------------+
+-------------------+ +-------------------+
| Virtual System | | Virtual System |
| system-id = S' | | system-id = S''|
| is-alias-id = S | | is-alias-id = S |
+-------------------+ +-------------------+
Figure 1. Advertising binding between all of a system's LSPs (both
modes)
When new extended LSP fragments are generated, these fragments should
be generated as specified in ISO 10589 [ISIS-ISO]. Furthermore, a
system SHOULD treat its extended LSPs the same as it treats its
original LSPs, with the exceptions noted in the following sections.
Specifically, creating, flooding, renewing, purging and all other
operations are similar for both original and extended LSPs, unless
stated otherwise. The extended LSPs will use one of the extended
system-ids configured for the router, in their LSP ID.
Extended LSPs fragment zero should be regarded in the same special
manner as specified in 10589 for LSPs with number zero, and should
include the same type of extra information as specified in 10589 and
RFC 1195 [ISIS-IP]. So, for example, when a system reissues its LSP
fragemnt zero due to an area address change, it should reissue all
extended LSPs fragment zero as well.
An extended LSP fragment zero MUST be generated for every extended
LSP set, to allow a router's SPF calculation to consider those
fragments in that set.
3.1.1 The Attached Bits
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The Attached (ATT) bits SHOULD be set to zero for all four metric
types, on all extended LSPs. This is due to the following: if a
Virtual System is reachable, so is its Originating System. It is
preferable, then, that an L1 IS chooses the Originating System and
not the Virtual System as its nearest L2 exit point, as connectivity
to the Virtual System has a higher probability of being lost (a
result of the extended LSP no longer being advertised). This could
cause unnecessary computations on some implementations.
3.1.2 The Partition Repair Bit
The Partition Repair (P) bit SHOULD be set to zero on all extended
LSPs. This is for the same reasons as for the Attached bits.
3.1.3 ES Neighbors TLV
ISO 10589 [ISIS-ISO] section 7.3.7 specifies inserting an ES Neighbor
TLV in L1 LSPs, with the system ID of the router. RFC 1195 [ISIS-IP]
relieves IP-only routers of this requirement. However, for routers
that do insert this ESN TLV in L1 LSPs (whether IP-only or OSI-
capable), then in an extended LSP, the ESN TLV should include the
relevant extended system-id. Furthermore, OSI-capable routers should
accept packets destined for this extended system-id.
3.2 Operation Mode 1 Additions
The following additions apply only to routers generating LSPs in Mode
1. Routers, which are configured to operate in Operation Mode 2,
SHOULD NOT apply these additions to their advertisements.
Under Operation Mode 1, adjacencies between the Original System and
its Virtual Systems are advertised using the standard neighbor TLVs.
The metric for these connections MUST be zero, since the cost of
reaching a Virtual System is the same as the cost of reaching its
Originating System.
To older implementations, Virtual Systems would appear reachable only
through their Originating System, hence loss of connectivity to the
Originating System means loss of connectivity to all of its
information, including that advertised in its extended LSPs.
Furthermore, the cost of reaching information advertised in non-
extended LSPs is the same as the cost of reaching information
advertised in the new extended LSPs, with an additional hop.
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+---------------------------------------------+
| Originating System |
| system-id = S |
| is-alias-id = S |
| ( extended system-ids = S', S'' ) |
+---------------------------------------------+
| /\ | /\
cost=0 | |cost=max-1 cost=0 | |cost=max-1
| | | |
\/ | \/ |
+-------------------+ +-------------------+
| Virtual System | | Virtual System |
| system-id = S' | | system-id = S''|
| is-alias-id = S | | is-alias-id = S |
+-------------------+ +-------------------+
Figure 2. Advertising connections to Virtual Systems under Operation
Mode 1
Under Operation Mode 1, only "leaf" information, i.e. information
that serves only as leaves in a shortest path tree, can be advertised
in extended LSPs.
When an extended LSP belonging to extended system-id S' is first
created, the original LSP MUST specify S' as a neighbor, with metric
set to zero. This in order to satisfy the two-way connectivity check
on other routers, as well as to consider the cost of reaching the
Virtual System S' the same as the cost of reaching its Originating
System. Furthermore, the extended LSP MUST specify the original
system-id as a neighbor, with metric set to (MaxLinkMetric - 1).
Where relevant, this adjacency SHOULD be considered as point-to-
point.
Note, that the restriction specified in ISO 10589 section 7.2.5
holds: if an LSP Number zero of the Originating System is not
present, none of that system's neighbor entries would be processed
during SPF, hence none of its extended LSPs would be processed as
well.
3.2.1 IS Neighbors TLV
An Extended LSP must specify only the Originating System as a
neighbor, with the metric set to (MaxLinkMetric - 1). Where
relevant, this adjacency should be considered as point-to-point.
Other neighbors MUST NOT be specified in an Extended LSP, because
those other neighbors would only specify the Originating System and
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not the extended system, and hence would not satisfy the bi-
directionality check in the SPF computation.
4. Purging Extended LSP Fragments
ISO 10589 [ISIS-ISO] section 7.3.4.4 note 21 suggests that an
implementation keeps the number of LSP fragments within a certain
limit based on the optimal (minimal) number of fragments needed.
Section 7.3.4.6 also recommends that an IS purge its empty LSPs to
conserve resources. These recommendations hold for the extended LSP
fragments as well. However, an extended LSP fragment zero should not
be purged until all of the fragments in its set (i.e. belonging to a
particular extended system-id), are empty as well. This is to ensure
implementations consider the fragments in their SPF computations, as
specified in section 7.2.5.
In Operational Mode 1, when all the extended LSP fragments of a
particular extended system-id S' have been purged, the Originating
System SHOULD remove the neighbor information to S' from its original
LSPs.
5. Modifications to LSP handling in SPF
This section describes modifications to the way extension-capable ISs
handle LSPs for the SPF computation.
When considering LSPs of an extension-capable IS (identified by the
inclusion of the IS Alias ID TLV), the original and extended LSPs are
combined to form one large logical LSP. If the LSP belongs to an IS
running Operational Mode 1, there might be adjacencies between the
original and extended LSPs. These are trivially ignored (since when
processing them the large logical LSP is already on PATHS), and
doesn't complicate the SPF. Furthermore, this check should already
be implemented (this scenario could occur on error, without this
extension)
If LSP fragment 0 of the original LSP set is missing, all of the LSPs
generated by that Originating System (extended as well) MUST NOT be
considered in the SPF. That is, the large logical LSP isn't
considered in the SPF. If an LSP fragment 0 of an extended LSP set
is missing, only that LSP set MUST NOT be considered in the SPF.
Note, that the above behavior is consistent with how previous
implementations will interpret Mode 1 LSPs.
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6. Forming Adjacencies
It should be noted, that an IS MUST use the system-id of the LSP that
will include a neighbor, when forming an adjacency with that
neighbor. That is, if a neighbor is to be included in extended LSP
S', then S' should be used as the system-id in IS Hellos [3] and IS-
IS Hellos when forming an adjacency with that neighbor. This is
regardless of the Operational Mode. Of course, in Mode 1 this means
that only the original system-id will be used when sending hellos.
7. Interoperating between extension-capable and non-extension-capable
ISs.
In order to correctly advertise link-state information under
Operation Mode 2, all ISs in an area must be extension-capable.
However, it is possible to not upgrade every router in the network,
if the extended information is not routing information, but rather
data that is of use to only a subset of routers (e.g. optical
switches using ISIS could carry optical-specific information in
extended LSPs)
It is possible to transition a live network, using the following
steps:
- Upgrade the routers, one-by-one, to run this extension in
Operation Mode 1. The other non-extension-capable routers will
interoperate correctly.
- When all routers are extension-capable, configure them one-by-one
to run in Operation Mode 2. All extension-capable routers
interoperate correctly, regardless of what mode they're run in.
8. Security Considerations
This document raises no new security issues for IS-IS.
9. Acknowledgments
The author would like to thank Tony Li and Radia Perlman for helpful
comments and suggestions on the subject.
10. References
[ISIS-ISO] ISO 10589, "Intermediate System to Intermediate System
Intra-Domain Routeing Exchange Protocol for use in Conjunction with
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the Protocol for Providing the Connectionless-mode Network Service
(ISO 8473)"
[ISIS-IP] RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual
environments", R.W. Callon, Dec. 1990
[ES-IS] ISO 9542, "End System to Intermediate System Routeing
Exchange Protocol for Use in Conjunction with the Protocol for
Providing the Connectionless-Mode Network Service (ISO 8473)", March
1988
[DOMAIN-WIDE] RFC 2966, "Domain-wide Prefix Distribution with Two-
Level IS-IS", T. Li, T. Przygienda, H. Smit, October 2000
[BCP14] RFC 2119, "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner, March 1997
[ISIS-CODES] Work in progress, "Reserved TLV Codepoints in ISIS", T.
Przygienda
11. Authors' Address
Amir Hermelin Email: amir@cwnt.com
Charlotte's Web Networks, Inc. Phone: +972 4 9592203
2 Ha'mada St. Fax: +972 4 9593325
POB 650
Yokneam, 20692
ISRAEL
Mike Shand, Email: mshand@cisco.com
Cisco Systems, Phone: +44 020 8824 8690
4, The Square,
Stockley Park,
UXBRIDGE,
Middlesex,
UB11 1BN,
UK
Stefano Previdi email: sprevidi@cisco.com
Cisco Systems, Inc. Phone: +32 2 7046590
De Kleetlaan 6A
1831 Diegem
Belgium
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