From: John Neiberger (neiby@xxxxxxxxxx)
Date: Sat Nov 10 2001 - 15:29:27 GMT-3
I learned something new today and I thought I'd share it. This may be
well-known but I hadn't run into it before.
I'm playing with redistributing IGRP into BGP. Here is the setup:
A----(IGRP)----B-----(eBGP)------C
Router B learns several routes from A via IGRP. As an example, let's say
they are 170.100.1.0, 170.100.2.0, 170.100.3.0 and 170.100.4.0.
Router B is configured to redistribute IGRP to BGP. Because auto-summary is
on by default, Router C learns 170.100.0.0/16.
I then decided I wanted B to forward a single more-specific prefix so I
added the statement 'network 170.100.4.0 mask 255.255.255.0'. This caused B
to send that prefix to C along with the summarized prefix.
The interesting thing about this is that I expected 170.100.4.0 to have a
zero metric on Router C, since it really isn't being redistributed at this
point. I assumed that the network statement would make it appear that this
route originated at router B.
Instead, Router C saw the new prefix but the IGRP metric from B was passed
along as the BGP metric, just like the summarized redistributed prefix!
So, what I found was that adding a network statement that includes a route
in the routing table that is learned from another protocol and is part of
redistribution will simply override auto-summary for that specific route and
it will forward the metric learned via the IGP.
Also, on router C that more-specific prefix has an origin code of ?, just
like the other redistributed routes.
I guess what I'm saying is this: if you want to advertise a summary address
as well as a few select more-specific prefixes while retaining the metric
learned via the IGP, you can leave auto-summary turned on an add explicit
network statements for your more-specific prefixes.
Again, this is probably not new to you guys but I thought this would have to
be done using aggregate-address along with a suppress map.
Regards,
John
This archive was generated by hypermail 2.1.4 : Fri Jun 21 2002 - 06:45:12 GMT-3