Network Working Group A. Hermelin
Request for Comments: 3786 Montilio Inc.
Category: Informational S. Previdi
M. Shand
Cisco Systems
May 2004
Extending the Number of
Intermediate System to Intermediate System (IS-IS)
Link State PDU (LSP) Fragments Beyond the 256 Limit
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a mechanism that allows a system to originate
more than 256 Link State PDU (LSP) fragments, a limit set by the
original Intermediate System to Intermediate System (IS-IS) Routing
protocol, as described in ISO/IEC 10589. This mechanism can be used
in IP-only, OSI-only, and dual routers.
Table of Contents
1. Introduction ................................................. 21.1. Keywords ............................................... 21.2. Definitions of Commonly Used Terms ..................... 21.3. Operation Modes ........................................ 31.4. Overview ............................................... 42. IS Alias ID TLV (IS-A) ....................................... 53. Generating LSPs .............................................. 63.1. Both Operation Modes ................................... 63.2. Operation Mode 1 Additives ............................. 84. Purging Extended LSP Fragments ............................... 105. Modifications to LSP handling in SPF ......................... 106. Forming Adjacencies .......................................... 117. Interoperating between extension-capable and non-capable ISs . 11
8. Security Considerations ...................................... 129. Acknowledgements ............................................. 12
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10. References ................................................... 1211. Authors' Addresses ........................................... 1312. Full Copyright Statement ..................................... 14
In the Intermediate System to Intermediate System (IS-IS) protocol, a
system floods its link-state information in Link State PDU (LSP) 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/IEC 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 increasing number of destinations 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.
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 BCP 14, RFC 2119
[BCP14].
This section provides definitions for terms that are used throughout
the text.
Originating System
A router physically running the IS-IS protocol. As this
document describes methods allowing a single IS-IS process to
advertise its LSPs as multiple "virtual" routers, the
Originating System represents the single "physical" IS-IS
process.
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Normal system-id
The system-id of an Originating System.
Additional system-id
An Additional system-id that is assigned by the network
administrator. Each Additional system-id allows generation of
256 additional, or extended, LSP fragments. The Additional
system-id, like the Normal system-id, must be unique throughout
the routing domain.
Virtual System
The system, identified by an Additional system-id, advertised
as originating the extended LSP fragments. These fragments
specify the Additional system-id in their LSP IDs.
Original LSP
An LSP using the Normal system-id in its LSP ID.
Extended LSP
An LSP using an Additional 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 Additional system-id, and
originated by the Originating System.
Extension-capable IS
An IS implementing the mechanisms described in this document.
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.
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- 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 is not compatible with previous implementations.
These modes are configured on a per-level and area basis. That is,
all LSPs considered in the same SPF instance MUST use the same Mode.
There is no restriction that an L1/L2 IS operates in the same mode,
for both its L1 and L2 instances. It can use Mode 1 for its L1 LSPs,
and Mode 2 for its L2 LSPs, or vice versa.
Mode 1 has the only advantage of being backwards compatible with
older implementations. It does have restrictions which are
considered drawbacks. Therefore, routers should operate in Mode 1
only if backwards compatibility is desired. Otherwise, it is
recommended to run in Mode 2.
Routers MAY implement Operational Mode 2 without supporting running
in Operational Mode 1. They will still interoperate correctly with
routers that support both modes.
Using Additional system-ids assigned by the administrator, the
Originating System can advertise the excess link-state information in
extended LSPs under these Additional 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 in its
fragment-0 a new TLV (IS Alias ID TLV) that contains the Normal
system-id and PN Number of the Original LSP created by the router.
Extension-capable ISs can then use this information and store the
original and extended LSPs as one logical LSP.
The only sections that deal only with Mode 1 additions are 3.2,
3.2.1, and 3.2.2. All other sections relate to both modes.
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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. It identifies the Normal
system-id of the Originating System.
The proposed IS Alias ID TLV is type 24, and its format is as
follows:
x CODE - 24.
x LENGTH - total length of the value field.
x VALUE -
No. of Octets
+-------------------+
| Normal system-id | 6
+-------------------+
| Pseudonode number | 1
+-------------------+
| Sub-TLVs length | 1
+-------------------+
| | 0-247
: Sub-TLVs :
: :
| |
+-------------------+
Normal system-id
The Normal system-id of the LSP set, as described in section 1.2
of this document.
Pseudonode number
The Pseudonode number of the LSP set. LSPs with the same Normal
system-id and Pseudonode number are considered in SPF as one
logical LSP, as described in section 5 of this document.
Sub-TLVs length
Total length of all sub-TLVs.
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Sub-TLVs
A series of tuples with the following format:
No. of Octets
+-------------------+
| Sub-type | 1
+-------------------+
| Length | 1
+-------------------+
| | 0-245
: Value :
: :
| |
+-------------------+
Sub-type
Type of the sub-TLV
Length
Total length of the value field
Value
Type-specific TLV payload.
For an explanation on sub-TLV handling, see [ISIS-TE].
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 running in either Mode 1 or Mode 2. Currently,
there are no sub-TLVs defined.
For a complete list of used IS-IS TLV numbers, see [ISIS-CODES].
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
extension-capable 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 of Original and
Extended LSPs.
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+---------------------------------------------+
| Originating System |
| system-id = S |
| is-alias-id = 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). S' and S'' are configured as Additional
system-ids.
When new extended LSP fragments are generated, these fragments should
be generated as specified in ISO/IEC 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 Additional
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 ISO/IEC 10589 for LSPs with number zero, and
should include the same type of extra information as specified in
ISO/IEC 10589 and RFC 1195 [ISIS-IP]. So, for example, when a system
reissues its LSP fragment 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. See section 5 for details.
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 (as a
result of the extended LSP no longer being advertised). This could
cause unnecessary computations on some implementations.
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ISO/IEC 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 Additional system-id. Furthermore, OSI-capable routers
should accept packets destined for this Additional system-id.
The overload bit should be set consistently across all LSPs, original
and extended, belonging to an Originating System, and should reflect
the Originating System's overload state.
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 from the Originating System to
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 |
+---------------------------------------------+
| /\ | /\
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. S' and S'' are configured as
Additional system-ids.
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 Additional system-id S' is first
created, the Original LSP MUST specify S' as a neighbor, with metric
set to zero. This is in order 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 Normal
system-id as a neighbor. The metric SHOULD be set to MaxLinkMetric -
1 (this is only for uniformity purpose, any metric greater than zero
is acceptable). This in order to satisfy the two-way connectivity
check on other routers. Where relevant, this adjacency SHOULD be
considered as point-to-point.
Note, that the restriction specified in ISO/IEC 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.
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
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those other neighbors would only specify the Originating System and
not the Virtual System, and hence would not satisfy the bi-
directionality check in the SPF computation.
If the Originating System is in the overload state, information in
the extended LSPs will not be processed by other routers in their SPF
computation. This is because in Mode 1, extended LSPs are reachable
only through adjacencies from the Original LSP. If this LSP has set
its OL bit, adjacencies will not be processed in the SPF computation.
This side effect should be taken into consideration when operating in
Mode 1.
ISO/IEC 10589 [ISIS-ISO] section 7.3.4.4 note 25 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 Additional 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 Additional system-id S' have been purged, the Originating
System SHOULD remove the neighbor information to S' from its original
LSPs.
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 does
not complicate the SPF. Furthermore, this check should already be
implemented (this scenario could occur on error, without this
extension).
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If LSP fragment 0 of the Original LSP set is missing or its
RemainingLifetime is zero, 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 is not considered in the SPF.
The original LSP fragments are identified when the is-alias-id value
is the same as the system-id of those LSPs. If an LSP fragment 0 of
an extended LSP set is missing or its RemainingLifetime is zero, only
that LSP set MUST NOT be considered in the SPF. These rules are
present to ensure consistent SPF results on Mode 1 and Mode 2 LSPs.
Note, that the above behavior is consistent with how previous
implementations will interpret Mode 1 LSPs.
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 Normal system-id will be used when sending hellos.
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 IS-IS could carry optical-specific information in
extended LSPs)
If a live network contains routers exceeding the 256 fragment limit,
and for some reason the upgrade has to be done incrementally, it is
possible to transition the 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 are run in.
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Implementations SHOULD support a configuration parameter controlling
the LSP origination behavior. The default value of this parameter
SHOULD correspond to the behavior described in [ISIS-ISO], i.e.,
neither of the two modes described in this document should be enabled
without explicit configuration when the router software is upgraded
with this extension.
[ISIS-ISO] "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/IEC 10589:2002, Second Edition.
[ISIS-IP] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[ISIS-TE] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic
Engineering (TE)", RFC 3784, May 2004.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[DOMAIN-WIDE] Li, T., Przygienda, T. and H. Smit, "Domain-wide Prefix
Distribution with Two-Level IS-IS", RFC 2966, October
2000.
[ISIS-CODES] Przygienda, T., "Reserved Type, Length and Value (TLV)
Codepoints in Intermediate System to Intermediate
System", RFC 3359, August 2002.
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RFC 3786 IS-IS LSP Fragments May 2004
Amir Hermelin
Montilio Inc.
1 Maskit St.
POB 12253
Herzelia, 46733
ISRAEL
Phone: +972 9 9511944
Fax: +972 9 9542430
EMail: amir@montilio.com
Stefano Previdi
Cisco Systems, Inc.
Via Del Serafico 200
00142 Roma
Italy
Phone: +39 06 5164 4491
EMail: sprevidi@cisco.com
Mike Shand
Cisco Systems
250, Longwater Avenue,
Green Park,
Reading,
RG2 6GB,
UK
Phone: +44 20 8824 8690
EMail: mshand@cisco.com
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