OSPF Stub Router Advertisement
Let a router tell others not to send transit traffic through it
Retana, A., Nguyen, L., White, R., Zinin, A., and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, DOI 10.17487/RFC3137, June 2001, <https://www.rfc-editor.org/info/rfc3137>.
@misc{rfc3137,
series = {Request for Comments},
number = 3137,
howpublished = {RFC 3137},
publisher = {RFC Editor},
doi = {10.17487/RFC3137},
url = {https://www.rfc-editor.org/info/rfc3137},
author = {A. Retana and L. Nguyen and R. White and A. Zinin and D. McPherson},
title = {{OSPF Stub Router Advertisement}},
pagetotal = 5,
year = 2001,
month = jun,
} RFC text
Contents
Network Working Group A. Retana
Request for Comments: 3137 L. Nguyen
Category: Informational R. White
Cisco Systems
A. Zinin
Nexsi Systems
D. McPherson
Amber Networks
June 2001
OSPF Stub Router Advertisement
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 (2001). All Rights Reserved.
Abstract
This memo describes a backward-compatible technique that may be used
by OSPF (Open Shortest Path First) implementations to advertise
unavailability to forward transit traffic or to lower the preference
level for the paths through such a router. In some cases, it is
desirable not to route transit traffic via a specific OSPF router.
However, OSPF does not specify a standard way to accomplish this.
1. Motivation
In some situations, it may be advantageous to inform routers in a
network not to use a specific router as a transit point, but still
route to it. Possible situations include the following.
o The router is in a critical condition (for example, has very
high CPU load or does not have enough memory to store all LSAs
or build the routing table).
o Graceful introduction and removal of the router to/from the
network.
o Other (administrative or traffic engineering) reasons.
Note that the proposed solution does not remove the router from the
topology view of the network (as could be done by just flushing that
router's router-LSA), but prevents other routers from using it for
transit routing, while still routing packets to router's own IP
addresses, i.e., the router is announced as stub.
It must be emphasized that the proposed solution provides real
benefits in networks designed with at least some level of redundancy
so that traffic can be routed around the stub router. Otherwise,
traffic destined for the networks reachable through such a stub
router will be still routed through it.
2. Proposed Solution
The solution described in this document solves two challenges
associated with the outlined problem. In the description below,
router X is the router announcing itself as a stub.
1) Making other routers prefer routes around router X while
performing the Dijkstra calculation.
2) Allowing other routers to reach IP prefixes directly connected
to router X.
Note that it would be easy to address issue 1) alone by just flushing
router X's router-LSA from the domain. However, it does not solve
problem 2), since other routers will not be able to use links to
router X in Dijkstra (no back link), and because router X will not
have links to its neighbors.
To address both problems, router X announces its router-LSA to the
neighbors as follows.
o costs of all non-stub links (links of the types other than 3)
are set to LSInfinity (16-bit value 0xFFFF, rather than 24-bit
value 0xFFFFFF used in summary and AS-external LSAs).
o costs of stub links (type 3) are set to the interface output
cost.
This addresses issues 1) and 2).
3. Compatibility issues
Some inconsistency may be seen when the network is constructed of the
routers that perform intra-area Dijkstra calculation as specified in
[RFC1247] (discarding link records in router-LSAs that have
LSInfinity cost value) and routers that perform it as specified in
[RFC1583] and higher (do not treat links with LSInfinity cost as
unreachable). Note that this inconsistency will not lead to routing
loops, because if there are some alternate paths in the network, both
types of routers will agree on using them rather than the path
through the stub router. If the path through the stub router is the
only one, the routers of the first type will not use the stub router
for transit (which is the desired behavior), while the routers of the
second type will still use this path.
4. Acknowledgements
The authors of this document do not make any claims on the
originality of the ideas described. Among other people, we would
like to acknowledge Henk Smit for being part of one of the initial
discussions around this topic.
5. Security Considerations
The technique described in this document does not introduce any new
security issues into OSPF protocol.
6. References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
7. Authors' Addresses
Alvaro Retana
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
EMail: aretana@cisco.com
Liem Nguyen
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
EMail: lhnguyen@cisco.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
EMail: riw@cisco.com
Alex Zinin
Nexsi Systems
1959 Concourse Drive
San Jose,CA 95131
EMail: azinin@nexsi.com
Danny McPherson
Amber Networks
48664 Milmont Drive
Fremont, CA 94538
EMail: danny@ambernetworks.com
8. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Motivation
In an OSPF network, routers forward packets destined for other routers and networks by computing shortest paths. Sometimes a router needs to stop forwarding transit traffic while remaining part of the network. This might happen when a router is CPU-bound, running low on memory, or being gracefully introduced to or removed from the network.
The RFC notes that simply removing the router from the network topology by flushing its router-LSA does not work because then other routers cannot reach the router's own IP addresses. The solution keeps the router in the topology but marks it as a stub router that should not carry transit traffic.
A router needs to say I am still here and you can reach me, but please do not send traffic through me to other places right now.
§2The Stub Router Solution
The solution modifies how the stub router advertises its router-LSA. Links of type three (stub links, which connect to directly attached networks) keep their normal interface costs. All other links get set to LSInfinity, a very large cost value. This causes other routers to avoid this router when computing paths through the network.
The technique is backward compatible. When the Dijkstra calculation discards links with LSInfinity cost, the router is completely avoided for transit. Older routers that do not discard such links may still use them, but alternate paths will always be preferred since they have lower costs.
The router says my local networks have normal costs but all my links to other routers cost infinity, so you should not route through me, but you can still reach my networks.
§3Compatibility with Older Routers
Different OSPF implementations handle infinity-cost links differently. Routers following RFC 1247 ignore links with LSInfinity cost entirely. Routers following RFC 1583 and later do not treat infinity-cost links as unreachable and will use them if necessary.
This does not create routing loops because if alternate paths exist, all routers prefer them to the infinity-cost path. If the stub router is the only way to reach a destination, routers that implement RFC 1247 skip it while RFC 1583 routers still use it. This inconsistency does not cause problems because the alternate paths and loop-free behavior are preserved.
Old and new routers interpret the infinity signal differently, but they both agree that other paths are better, so the network still works correctly.