An OSPF route won't appear in the routing table though it exists in LSDB and forward address is reac

Tomohide

I encountered a situation where an OSPF External route won't appear in the routing table even though it exists in LSDB and the forward address is reachable.

 

My understanding is OSPF Type-5 external routes won't get into the routing table if:

  - Forward Address is set AND

  - The forward address is not reachable

 

To prove the above, I set up the following OSPF areas where a router in Area 1 receives a default route via BGP.

[BGP] -- [Area 1] -- [Area 0] -- [Area 2]

*Note OSPF is enabled on the WAN interface in addition to BGP though there's no OSPF neighbor in that direction for the sake of experiment. (Without it, the Forward Address is not set.)

 

In this state, I checked the routing table on a router in Area 2 and, as expected, found the default route installed as OSPF E2.

 

Next, I set up a filter on ABR between areas 0 and 1 so that the forward address for the default route won't be propagated into Area 0.

area 1 range 12.12.12.0 255.255.255.0 not-advertise  (12.12.12.0 is the WAN link address and forward address for the default route)

 

After this, the default route is gone from the routing table although it's still in LSDB.  That's because there's no reachability to the Forward Address.

In order to make the Forward Address reachable, I added a static route for the forward address on every router in Area 0 and Area 2 (except for the ABR between Areas 1 and 0).  I thought this will bring the default route back into the routing table on Area 2 routers, but it DIDN'T.

I can ping and traceroute to the forwarding address but still the default route won't appear in the routing table.

Can anyone explain why?

Also, how can I bring the default route back into Area 2 routers without removing the filter (i.e., area 1 range 12.12.12.0 255.255.255.0 not-advertise).

opadesa

I think there's another condition that the FA must be only be learned as an O or O IA route for the OSPF E route to be installed in the route table. 

martino

I think that answer to your question is in the RFC.

Static route doesn't meet the IA / O requirement.

ciprian_s

Same thing would happen if, for example, the FA is set on an interface directly connected.

You may want to take a look at the following post, I presented the FA topic with an example, referencing RFC 2328 as well.

http://ieoc.com/forums/t/30090.aspx

 

Tomohide

Hey guys.

Thanks a lot!  I understand the reason now [:D]

ciprian_s

Hi,

I was going through OSPF ATC R&S v5 version and Brian pointed quite a good resource on this topic. For further reading,

 you may want to take a look at the following document, good resource :

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7112-26.html

 

 

Tomohide

Actually, I recently implemented an OSPF network and somehow some routes didn't get into the routing table though they're in LSDB.  I checked the LSDB and found the "Advertising Router" addresses were not in the routing table (the Forward Addresses were set to 0.0.0.0 this time).

This kind of problem really happens in the field and we can't afford not to master this "Not-in-RIB-though-it's-in-LSDB" thing. [:P]

I've been busy and haven't gotten around to solving the problem yet, but the link may help me identify the culprit.  Thank you again.