From: Bob Sinclair (bsin@cox.net)
Date: Tue Jun 01 2004 - 20:43:54 GMT-3
Peng,
BGP advertises only its best route. For example, if R1 and R2 are border
routers in the same AS and they both learn an external prefix, they will
each advertise that prefix to the other, and each border router will have
both an internal and an external route to the prefix.
If you then set local preference so that the AS prefers the exit through R1,
then R2 will no longer send the route it learned externally to R1. Since R2
prefers the route learned from R1, and R2 only sends its preferred route,
R1 will end up with just one route (the preferred external route) and R2
will have two routes (the internal route from R1, and the less-preferred
external route).
HTH,
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Peng Zheng" <zpnist@yahoo.com>
To: <ccielab@groupstudy.com>
Sent: Tuesday, June 01, 2004 6:27 PM
Subject: BGP local-preference
> Three Router, R1, R2, R3, all in AS 100. Fully meshed.
>
> R1 connect to BB1 (in AS 200),
> R3 connect to BB2 (also in AS 200).
>
> They both receive some prefix, R3 also generates a
> summary route. Ask R1 make R1 and R2 to prefer R1 to
> subnet, and prefer R3 to the summary.
>
> Before I configure, R1, R2 and R3 can see all routes
> in their BGP table, each of them are from both R1 and
> R3.
>
> Then I set up those subnet from BB1 (on R1) with
> local-preference 200, Now from R2 and R1, I can only
> see (in BGP table) those subnets from R1. If I
> shutdown the peer between R2 and R1, R2 can see the
> subnets from R3. Why I can not see those subnets
> from R3 on R1 and R2 after I set the local-preference?
>
>
>
>
>
>
>
>
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This archive was generated by hypermail 2.1.4 : Sat Jul 03 2004 - 19:40:31 GMT-3