Monday, June 22, 2009

Follow-up to OSPF Boundary Tricks

I recently found a new trick that was posted in Shivlu's blog on how to eliminate OSPF External routes from entering the domain:

The idea behind this method is to create a virtual loopback interface that has the same IP subnet of the customer network beyond the connected interface. You then apply the 'ip ospf network point-to-point' command to that loopback interface to avoid the IOS default of a stub host (it would only advertise the /32 address of the interface). A local static route for the customer network is then pointed out the real interface (use of route-maps is optional to block this static route if there are other statics that need to be redistributed into OSPF).

The router is then able to advertise the customer route, throughout the OSPF domain, that it knows about it 'internally'. Once the packet reaches the router, the router will use the static route to correctly route it.

Tip of the hat to Shivlu for this trick!


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