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
>
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