A question regarding IS-IS and BGP interoperability.
A network runs IS-IS and consists of two areas. Each area has one L1L2
router and a couple of L1 routers. L1L2 routers from the two areas are
connected to one another.
Each L1L2 router has a BGP session with each L1 router in its area and
acts as a route reflector for them. Both L1L2 talk BGP to each other as
well.
+--------------------------------+
| Area1 Area2 |
| ~~~~~ ~~~~~ |
| +------+ +------+ |
| | L1L2 |-----| L1L2 | |
| +------+ +------+ |
| ||| ||| |
| +----+ +----+ |
| | L1 |+ | L1 |+ |
| +----+|+ +----+|+ |
| +----+| +----+| |
| +----+ +----+ |
+--------------------------------+
Suppose, an L1 router in Area 1 advertises some network via BGP.
An L1 router in Area 2 receives this advertisement with next-hop set to
a loopback IP of the Area 1 router, and the only way for it to reach this
IP is via default gateway (because it comes from another area....). Hence,
it will not accept the advertisement.
Possible solutions I've thought about:
1. Leak Area 1 aggregate into Area 2 and vice versa.
2. Manually set the next-hop to L1L2 router's loopback for all prefixes
learned from another area.
What is the common practice for such configurations, that is, multiple
IS-IS areas with BGP? How the next-hop issue should be dealt with?
I would also appreciate pointing me to some resources regarding IS-IS and
BGP interoperability.
Thanks,
Elijah
On Sun, 17 Jun 2001, Elijah Kagan wrote:
>
> Suppose there is a router (a Cisco router, of course) that learns nothing
> but a default gateway from its IGP. It also has an iBGP session with its
> default router and receives the full Internet routing table. The default
> router acts as a route-reflector.
>
> Now here is the problem. The next-hop of every BGP prefix it learns is
> reachable via IGP's default gateway, but for some reason it is not good
> enough, BGP marks it as inaccessible and disregards the prefix.
>
> This situation could appear in L1 router that learns a prefix originating
> form a different IS-IS area.
>
> Please advise....
>
> -- elijah
>
>
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