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

Who said the rules of networking had to be consistent? ;)

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Jared Scrivener
CCSI #30878, CCIE3 #16983 (R&S, SP, Security)
www.MicronicsTraining.com
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On Sat, Feb 13, 2010 at 11:04 PM, <Keegan.Holley_at_sungard.com> wrote:
> 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?
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
> >
> >
> >
> >
> >
> >
> >
> >
>
>
> --
>
> Sent from Doha, Qatar
>
>
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> _______________________________________________________________________
> Subscription information may be found at:
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>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>
>
>
-- 
Cheers,
Jared Scrivener
CCSI #30878, CCIE3 #16983 (R&S, SP, Security)
www.MicronicsTraining.com
Sr. Technical Instructor
YES! We take Cisco Learning Credits!
Training And Remote Racks available
LinkedIn:www.linkedin.com/in/jaredscrivener
Blogs and organic groups at http://www.ccie.net