networking n stuff

Thursday, August 31, 2017

The Curious Case Of OSPF NSSA LSA on GRE Tunnel

Today I encountered a case which is really easy when you know what exactly to look for but seems puzzling at the first glance.

This is what our topology looks like:

We have two routers connecting two locations with OSPF configured on each. The primary connection is via GRE tunnel while backup connection is just a direct connection (say we have dark fiber or L2VPN between these locations). OSPF neighbor relations are established on both links.

Nodes exchange routes between each other and everything goes on just fine..until the primary connection goes down. Then some of the routes R2 sends to R1 are seen on R1 as connected via the backup link..and some are still seen as connected via tunnel interface, though OSPF neighbor is already considered dead on this link.

Sounds intriguing?