Re: BGP/OSPF router-id mismatch and the BGP synchronization

Check this out. http://www.internetworkexpert.com/rfc/rfc1403.txt

> 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
>
>
>
> Varadhan [Page 5]
>
> RFC 1403 BGP OSPF Interaction January 1993
>
>
> 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.
>
>
On Sat, Feb 13, 2010 at 6:16 AM, Marcel Lammerse <m.lammerse_at_mac.com> wrote:

> Hi all,
>
> I know this is not common these days, as BGP synchronization is turned off
> by default in recent ios versions. However, I was wondering if anyone knows
> why a Cisco router considers an OSPF route not synchronized, if the OSPF
> router id and BGP router id don't match.
>
> Is this an implementation-specfic thing or is there some protocol design
> thinking behind it?
>
>
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