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RE: BGP quandary
From: Leo Nelson <lnelson () Axient com>
Date: Wed, 11 Oct 2000 12:00:35 -0700

I do not claim to be a BGP guru but here is my take.  If my take is way of
base I would like to here from our BGP guru's here

-----Original Message-----
From: Timothy Brown [mailto:tcb () ga prestige net]
Sent: Wednesday, October 11, 2000 10:15 AM
To: nanog () merit edu
Subject: BGP quandary

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.

!# This is a bit confusing to me p1(AS12345) is p1 no matter what rtr
connects to it or where it connects. Same goes for p2(As12346). 

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).

!# This may be difficult to do if you want any type of redundant pathing.
Both swr's will need a peering session (tcp connection) to both rtr's for
redundancy(unless you don't care). Therefore I would suggest iGBP peering
betwween the rtr's and swr's and not worry about the swr's making a
decision. You can use dampening if you are worried about excessive cpu

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 only way to get one "best path" pushed is to have only one peer, igrp
or egrp or default route to the eBGP rtr. Ohterwise the routing protocol
will make a decision on what route to inject into it's table based on the
metric if more than one is recieved.

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?


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  • BGP quandary Timothy Brown (Oct 11)
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