Multihome design, local default routing with OSPF

[mstubbers]

We are establishing a multihome ISP link with a single service provider. The service is managed by the provider, so I don't have to deal with any BGP configurations. Which means we will have their routers on site. We are adding our own routers just inside the providers and will then feed to a pair of PIX515's. My goal is to esablish OSPF routing on my routers which would then feed two default routes to my PIX's to enable load balancing when both T1's are active.

My question is around OSPF monitoring for a failure in either of the ISP T1's and stopping the default route announcement to my PIX for that leg. Does OSPF have the capability to monitor a "next-hop" interface that is not inside of its area to determine when to fail a routing announcement?

ISP_RTRA <<<T1>>> ISP_RTRC<<>>RTRA<<>>PIXA

| |

ISP_RTRB<<<T1>>>ISP_RTRD<<>>RTRB<<>>PIXB

So ISP_RTRC, ISP_RTRD, RTRA and RTRB would be inside the OSPF area. We would want to verify the reachability of ISP_RTRA and ISP_RTRB to have RTRA and RTRB announce the default to PIXA and PIXB.

Or is there a better method for us to detect a BGP session failure on the ISP managed equipment?

Thanks!

[mmolina2]

You can configure a conditional OSPF default route on both of your routers. This may not be perfect but it should work.

Let's assume the /30 subnet for the ISP_RTRC to ISP_RTRA T1 is 1.1.1.0 /30

Let's assume the /30 subnet for the ISP_RTRD to ISP_RTRB T1 is 2.2.2.0 /30

If RTRC and RTRD are redistributing these directly connected T1 subnets into the OSPF process, you can configure the following:

RTRA#

router ospf 1

network x.x.x.x x.x.x.x area x

default-information originate route-map conditional

route-map conditional permit 10

match ip address 1

access-list 1 permit 1.1.1.0 0.0.0.3

This config will check RTRA's route table for the existence of the 1.1.1.0 /30 subnet before RTRA generates the default to the PIX. Obviously if you lose your T1 for this subnet, RTRA will no longer generate the default since it will lose the 1.1.1.0 /30 advertisement from ISP_RTRC. Repeat the steps for RTRD for the 2.2.2.0 /30 subnet.

As always, I could be wrong. Good luck !