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

Couple this with the fact that I can't even remember the last time I came
across a router with synchronization turned on and this becomes a nice
little gotcha. It's strange that the BGP process doesn't consider a route
good if it's protocols are using different router ID's. Shouldn't it
simply "trust" the IGP to make the right decision? Matching router ID's
don't buy you anything by themselves. There is still the potential for
loops if the BGP path doesn't match the OSPF one. Matching isn't even
possible with protocols such as RIP and EIGRP so that makes it even
stranger. Am I missing something here?

From:
Divin Mathew John <divinjohn_at_gmail.com>
To:
Marcel Lammerse <m.lammerse_at_mac.com>
Cc:
Cisco certification <ccielab_at_groupstudy.com>
Date:
02/12/2010 11:17 PM
Subject:
Re: BGP/OSPF router-id mismatch and the BGP synchronization rule
Sent by:
<nobody_at_groupstudy.com>

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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>
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