Network Working Group Alvaro Retana
Internet Draft Liem Nguyen
Expiration Date: May 2001 Russ White
File name: draft-ietf-ospf-stub-adv-01.txt Alex Zinin
Cisco Systems
Danny McPherson
Amber Networks
November 2000
OSPF Stub Router Advertisement
draft-ietf-ospf-stub-adv-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
In some cases, it is desirable not to route transit traffic via a
specific OSPF router. However, OSPF [RFC2328] does not specify a
standard way to accomplish this. This memo describes a backward-
compatible technique that may be used by OSPF implementations to
advertise unavailability to forward transit traffic or to lower the
preference level for the paths through such a router.
1 Motivation
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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 bene-
fits 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
either be still routed through it.
2 Proposed Solution
The solution described in this document solves two challenges associ-
ated 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 per-
forming the Dijkstra calculation.
2) Allowing other routers to reach IP prefixes directly con-
nected 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.
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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 Implementation details
To simplify the implementation of the described technique, it may be
useful for OSPF routers to keep two versions of their router-LSAs.
One version (public) is installed in the LSDB and sent to the neigh-
bors, while another version (internal) is used for local routing
table calculation. When the router announces itself as stub, it con-
structs the public version as indicated in Section 2, but the inter-
nal version is constructed as in standard OSPF [RFC2328]. When the
router does not announce itself as stub, both versions are con-
structed as in standard OSPF and would both yield the same result,
hence it is not necessary (though acceptable) to keep two versions of
the LSAs at this point.
4 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 LSInfin-
ity 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.
5 Acknowledgements
The authors of this document do not make any claims on the original-
ity of the ideas described. Among other people, we would like to
acknowledge Henk Smit for being part of one of the initial discus-
sions around this topic.
6 Security Considerations
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The technique described in this document does not introduce any new
security issues into OSPF protocol.
7 References
[RFC2328]
J. Moy. OSPF version 2. Technical Report RFC 2328,
Internet Engineering Task Force, 1998.
ftp://ftp.isi.edu/in-notes/rfc2328.txt.
[RFC1247]
J. Moy. OSPF version 2. RFC 1247,
Internet Engineering Task Force, 1991.
ftp://ftp.isi.edu/in-notes/rfc1247.txt.
[RFC1583]
J. Moy. OSPF version 2. RFC 1583,
Internet Engineering Task Force, 1994.
ftp://ftp.isi.edu/in-notes/rfc1583.txt.
8 Authors' addresses
Alvaro Retana
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
e-mail: aretana@cisco.com
Liem Nguyen
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
e-mail: lhnguyen@cisco.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
e-mail: riw@cisco.com
Alex D. Zinin
Cisco Systems
150 West Tasman Dr.
San Jose,CA 95134
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E-mail: azinin@cisco.com
Danny McPherson
Amber Networks
2465 Augustine Drive
Santa Clara, CA 95054
e-mail: danny@ambernetworks.com
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