Routing

Hi Experts

VR-1 has ospf neighbour with some router say R. I exported OSPF routes from VR-2 to VR-1 but I noticed that VR-1 has now these OSPF routes from VR-2 but it is not advertising these OSPF routes to router R. I have to make a routing policy accepting these ospf routes and apply as a export to OSPF in VR-2.

Could some one explain although VR-2 has OSPF exported routes from VR-2 but it is not advertising these OSPF routes further without routing policy?

Thanks

Hello there,

OSPF does not advertise or store routes, it advertises and stores LSAs in LSDB. Only after SPF is run, the actual routes are calculated and inserted into route table.

Back to your example: VR-2 has routes injected in its route table by means of policy. That's not enough for OSPF inside VR-2, because the LSDB for OSPF process inside VR-2 does not have knowledge about these imported-from-VR-1 routes/prefixes:

a/ these routes were not received as LSA from OSPF neighbor visible in VR-2

b/ there were no connected OSPF-enabled interface subnets inside VR-2 which matched these routes.

Only when you added a policy to redistribute imported-from-VR-1 routes into OSPF, only then these routes made its way into LSDB for OSPF inside VR-2.

HTH

Rgds

Alex 

Thanks Dear...It makes sense. Just one more thing that if I imported  any local/direct subnet from the say VR-1 to VR-2 and If try to ping that IP from VR-2, I am not able to ping unless I also imported any local/direct subnet from VR-2 to VR-1. Why thats so?

Because if the the ping is sourced from inside VR-2 then the ping is source IP@ is one of the VR-2 local interface' IP@. 

If you wish your ICMP Echo reply to be returned from VR-1 to VR-2, you have to make VR-2' direct routes known in VR-1.

Thanks for the explaination