John et al: The first part of my message said that this is a particular problem with route reflectors. Don't know what I'm doing but the first line of my messages gets chopped frequently.
Just didn't want people to think the BGP/OSPF requirement was arbitrary, though it seems to be based on an obsolete RFC. As you found, it is real in cisco BGP/OSPF configurations. Thanks to Howard and Pete for updating me on the RFC's. Cheers, Fred. John Neiberger wrote: > > Fred, > > Thanks, that's exactly what I was looking for, although it appears that > the first part of your response was chopped off for some reason. > > John > > >>> "Fred Ingham" 12/28/01 12:19:59 PM >>> > reflectors. > > The requirement for the BGP/OSPF identifier is stated in RFC 1364: > > "Varadhan [Page > 4] > > RFC 1364 BGP OSPF Interaction September > 1992 > > > 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. Consider the scenario in which 3 routers, 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 use the AS_PATH of the route announced by > the ASBR, whose BGP Identifier is the same as the OSPF routerID > corresponding to its route for network X. > > ii. It will be convenient for the network administrator looking > at > an ASBR to correlate different BGP and OSPF routes based on > the identifier." > > Cisco issued a field notice on the problem where there was a route > reflector. (Can't locate it in my pile(s)) > > In One Tech Note (BGP Best Path Selection Algorithm): > > "Paths marked as "not synchronized" in the show ip bgp > output. If BGP synchronization is enabled, which it is by default in > Cisco IOS. Software, there must be a match for the prefix in the IP > routing table in order for an internal (iBGP) path to be considered a > valid path. If the matching route is learned from an OSPF neighbor, > its > OSPF router ID must match the BGP router ID of the iBGP neighbor. Most > users prefer to disable synchronization using the no synchronization > BGP > subcommand." > > The comparison can be done using the sh ip bgp xxx.yyy.zzz.aaa and > show > ip ospf data. > > HTH, Fred. > > John Neiberger wrote: > > > > We discovered something on the CCIE list recently and I'm > > wondering if anyone might be able to explain the reasoning > > behing this behavior. > > > > BGP synchronization rules require that if an iBGP peer is to > > advertise a route learned via iBGP, it must have that prefix > > *and* the next hop for that route in the routing table already. > > > > An interesting added complexity to this occurs if your IGP is > > OSPF. If the router in question has learned these prefixes via > > OSPF, then the advertising router ID in the OSPF database must > > match the router ID of the iBGP peer that advertised the route. > > > > Has this behavior caused any problems for any of you? Do you > > know why the synchronization rules have a special case for OSPF > > and not other routing protocols? > > > > I was working with someone else on a practice lab and we ran > > into this issue. We were both going nuts trying to figure out > > why the iBGP routes weren't synchronizing and this turned out > > to be the cause. > > > > Any thoughts? > > > > Thanks, > > John > > > > ________________________________________________ > > Get your own "800" number > > Voicemail, fax, email, and a lot more > > http://www.ureach.com/reg/tag Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=30409&t=30126 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
