Re: [nsp] how to fool the SPF in OSPF?

From: David Bergum (bergum@cisco.com)
Date: Mon Jun 11 2001 - 15:59:42 EDT


>>>>> On Mon, 11 Jun 2001 15:45:48 -0400 (EDT), <jlewis@lewis.org> said:

  jlewis> Given a network such as:
  jlewis> -----------------------------------
  jlewis> | routerA------T3------routerC |
  jlewis> |___ | _________________ | __| area 0
  jlewis> | | | | | |
  jlewis> | T1s | | T3 |
  jlewis> | | / | | |
  jlewis> \ | / /-------------\ \ | /
  jlewis> routerB-----100bt------routerD
  jlewis> \-------------/
  jlewis> area 1

  jlewis> how do I get around the T1's connecting A and B being the shortest path
  jlewis> and get B to send traffic to A via D unless one of the T3's above is down?
  jlewis> I can't just use static routes on B, because it won't know if a T3 is
  jlewis> down. I've tried playing with "ip ospf cost" on the T1's but B still
  jlewis> prefers them.

It's hard! OSPF prefers intra-area routes over inter-area routes, despite
the cost. Make it all area 0 and your active path will be the lowest
cost. Choosing area boundaries is a hard thing to do.

Dave.



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