OSPF Summary Routes and BGP

Recently I was in a situation where I needed to advertise some OSPF routes created using the area range command into BGP. When advertising routes into BGP there are a few considerations:

  • Does the routing table know the exact route you’re trying to advertise into BGP?
  • Is any route filtering being performed? Don’t forget to check at the source of the BGP route and the destination it’s being advertised to!
  • Is soft-reconfiguration supported on the software you’re running?
  • Will you need to do a “clear ip bgp neighbor”? Seems IOS 12.4 doesn’t require it but 12.2 does. I tested 12.4 on GNS3, and 12.2 on a live 6500.

Using the area range command will automatically generate an OSPF intra-area route to Null 0 IF the router the command is issued on is an ABR. This is visible here:

Switch#sh ip route 10.253.0.0 255.255.240.0 
Routing entry for 10.253.0.0/20
Known via "ospf 1", distance 110, metric 0, type intra area
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 0, traffic share count is 1

This route will not be created on a non-ABR router, so watch out if you’re using single area OSPF. You’ll need to create a static null route to the summary range  you’re trying to advertise on a non-ABR router.

Anyway, I was trying to clear up whether BGP would take an auto-generated null 0 route for a network advertisement. Seems it will!

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