Wednesday, August 3, 2011

ISIS default route redistribution

I run into the following problem: the default route is not redistributed from BGP to ISIS during the redistribution process. It turned out that this is normal:

You can force a default route into an IS-IS routing domain. Whenever you specifically configure redistribution of routes into an IS-IS routing domain, the Cisco IOS software does not, by default, redistribute the default route into the IS-IS routing domain. The following feature allows you to force the boundary router to redistribute the default route or generate a default route into its L2 LSP. You can use a route-map to conditionally advertise the default route, depending on the existence of another route in the router's routing table.

To generate a default route, perform the following task in router configuration mode:



Force a default route into the IS-IS routing domain.

default-information originate [route-map map-name]

So we need a route-map to generate a default route conditionally. I did the following:

ip prefix-list DEFAULT-ROUTE seq 10 permit

ip access-list standard LOCAL-BGP-NEXTHOP
 permit *az eBGP peer IP cime (next hop)*

 match ip address prefix-list DEFAULT-ROUTE
 match ip next-hop LOCAL-BGP-NEXTHOP

router isis
 default-information originate route-map ISIS-DEFAULT-INFORMATION-ORIGINATE

It will generate a default route only if it's already in the routing table, and the local BGP peer is the next hop for it. The backup router won't generate one, cause the default route's next hop is not its neighbor.

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