Hi,
I'm looking at a current issue where we have 2 x ASBR routers also peering with BGP neighbors where the default-route is received. Local preference configuration on the ASBR routers influence the preferred outbound path and ASBR-1 installs the eBGP default in to it's routing table. ASBR 2 has an iBGP entry for the default route as they are peering with each other.
The same 2 ASBR routers are configured with the 'default-information originate' command in OSPF. These ASBRs then both connect to an upstream switch configured to run ospf (everything is in area0)
What appears to be happening is that ASBR-1 with the eBGP default in its routing table is starts advertising the default to it's OSPF neighbors (ASBR-2 and the downstream switch). ASBR-2 then installs this OSPF learnt default in its routing table as it has a better AD than the iBGP learnt route).
Now that ASBR-2 has the both the iBGP leant default and the OSPF learnt default entries it prefers the OSPF route learnt from ASBR-1 and installs it in the routing table. ASBR-2 at this point starts advertising the default route due to having the default-information originate command and the valid OSPF route in its routing table.
Is this normal behaviour?
this behavious is leading to a routing loop because both ASBR-1 and ASBR-2 are now advertising the default route to the downstream switch. The downstream switch now has equal cost default routes via either ASBR. Due to link costs ASBR-2's best path to the default (advertised by ASBR-1) is via the downstream switch. This results in some traffic from ASBR-2 following the default to the downstream switch which then forwards some of the traffic back creating a loop.
I'd like add that this issue isn't my own creation and I have a couple of solutions in mind. More curious if this is a bug or just down to misconfiguration.
One more point is that ASBR-2 will at times stop advertising the default. When I issue the show ip database external 0.0.0.0 detail (or something similar, not at a device to check syntax) at this time there is only one Type-5 LSA which is via ASBR-1.
Thanks
