From: cciestudy (cciestudy@mid-world.net)
Date: Mon May 19 2008 - 13:23:19 ART
I was not aware there was an RFC behind this behavior.
So, the fundamental reason they need to be the same is that the BGP process
doesn't inherently know the ASBR location? What info was not transferred
via the redistribution?
-----Original Message-----
From: Paul Cosgrove [mailto:paul.cosgrove@heanet.ie]
Sent: Monday, May 19, 2008 4:02 AM
To: cciestudy
Cc: ccielab@groupstudy.com
Subject: Re: BGP sync and OSPF router-id
The rule about using the same OSPF and BGP router ID is taken from
RFC1403, which defines BGP and OSPF interaction:-
3. BGP Identifier and OSPF router ID
The BGP identifier MUST be the same as the OSPF router id at all
times that the router is up.
This characteristic is required for two reasons.
i Synchronisation between OSPF and BGP
Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3,
belong to the same autonomous system.
+-----+
| RT3 |
+-----+
|
Autonomous System running OSPF
/ \
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
Both RT1 and RT2 have routes to an external network X and
import it into the OSPF routing domain. RT3 is advertising
the route to network X to other external BGP speakers. RT3
must use the OSPF router ID to determine whether it is using
RT1 or RT2 to forward packets to network X and hence build the
correct AS_PATH to advertise to other external speakers.
More precisely, RT3 must determine which ASBR it is using to
reach network X by matching the OSPF router ID for its route
to network X with the BGP Identifier of one of the ASBRs, and
use the corresponding route for further advertisement to
external BGP peers.
ii It will be convenient for the network administrator looking at
an ASBR to correlate different BGP and OSPF routes based on
the identifier.
Regards,
Paul.
cciestudy wrote:
> After struggling with he IE lab 13 10.3 BGP synchronization, can anyone
> explain the reason you need the router-id in BGP and OSPF to be the same
in
> order to get the BGP route to synchronize?
>
>
>
> I found referencing to elsewhere, but none gave a good explanation as to
> why?
>
>
>
> Thanks
>
>
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