Re: BGP sync and OSPF router-id

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Mon May 19 2008 - 06:02:14 ART

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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