RFC 3137 (rfc3137) - Page 2 of 5

OSPF Stub Router Advertisement

RFC 3137             OSPF Stub Router Advertisement            June 2001


   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).










Retana, et al.               Informational