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BGP quandary

  • From: Timothy Brown
  • Date: Wed Oct 11 13:20:25 2000

This may be an obvious answer, but, like some obvious answers, you just
never get the clue, even though you struggle over it.

I have four routers in a network.  Two of the routers have no external
peers, two of them have two peers a piece.  The diagram looks like this:

       p1(e) p2(e)      p1(e)   p2(e)
      /     /          /       /
     .-------.        .--------.
     | rtr_1 |        | rtr_2  |
     `-------'        `--------'
        | (i)             | (i)
     .-------.        .--------.
     | swr_1 |--------| swr_2  |
     `-------'        `--------'
        |               |
          |         |          |
          fw        fw         fw

p1 (AS12345) and p2 (AS12346) are the same AS across both routers, but
connected to different pops.  so on rtr_1, p1 may go to sjc, but on rtr_2,
p1 may go to lax, even though it's the different AS.

My quandary is this:  I want to advertise my best path to swr_1 and swr_2.
the line between swr_1 and swr_2 is a trunk; e.g., they're one piece of
broadcast media, but they both have routing cards in them to process IP
packets (and BGP).  my problem is, rtr_1 is going to select a best path from
p1 and p2, and advertise it to whoever his peers are.  rtr_2 will do the
same, but with the additional caveat that rtr_1 may advertise his best path
to rtr_2, giving rtr_2 three paths to choose from in this example (assuming
p1 and p2 only advertise one path to me a piece for the same netblock).

further complicating the issue, swr_1 and swr_2 have to accomplish the same
task.  i don't want those two boxes to have to deal with selecting a best
path; I would like the best path pushed down from rtr_1 and rtr_2, but i'm
not sure where i'm pushing it.

the fw device will route to the swr devices, probably via HSRP, but if they
don't do that via HSRP, they have to select an additional best path (based
on network load) via IGP, deciding which swr to connect to (assuming they
are capable of that).  the swr devices will have to identify a way to route
externally, which i'm also not sure of.

I can see several options, all of which have significant disadvantages and
weird routing behaviour.

does anyone have any suggestions on how to solve this situation?