Multiple OSPF E2 routes.

Hi all,

I have a basic topology, where im redistributing some subnets into OSPF. They are just redistributed pretty standard, with normal E2 route types. However, im doing this in two places. My thought would be that since its done in two places, and the internal cost doesnt increase (since its E2), why dont I see two equal cost entries in my routing table?

I do see them in my ospf database:

192.168.12.0 1.1.1.1 1301 0x80000001 0x009393 0
192.168.12.0 3.3.3.3 1300 0x80000001 0x0057C7 0

but in my routing table i only see:
O E2 192.168.12.0/24 [110/20] via 192.168.14.1, 00:08:37, Serial0/0

This is weird to me, since they have the same metric value.
Something im missing?

Update:
I figured out that even though the metric is the same, the LSA that will be installed in the routing table is the one from the ASBR with the lowest metric.
a "sh ip ospf border-routers" shows the culprit:

i 1.1.1.1 [64] via 192.168.14.1, Serial0/0, ASBR, Area 0, SPF 13
i 3.3.3.3 [128] via 192.168.45.5, Serial0/1, ASBR, Area 0, SPF 13

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Comments

EdTheLad

If there are two paths with an equal type 2 metric, type 1 will be used.If you do a show ip route x.x.x.x for a learned ospf route, you should see a forwarding metric entry, this is actually the type 1 metric.

kpjungle

EdTheLad wrote: »

If there are two paths with an equal type 2 metric, type 1 will be used.If you do a show ip route x.x.x.x for a learned ospf route, you should see a forwarding metric entry, this is actually the type 1 metric.

Yeah.. as long as the forwarding info for the routes are both 0.0.0.0 (as when they are redistributed), the metric to the ASBR's are taken into account.

I used "sh ip ospf database external" to check out the forwarding.

The Effects of the Forwarding Address on Type 5 LSA Path Selection [IP Routing] - Cisco Systems

The above helped to figure it out.

PStefanov

Thanks for the link. By the way, I encountered something I hadn't noticed before - if the external link (the link to a router that you receive an external route from) that runs OSPF is not an OSPF network type of either broadcast or non-broadcast, the forwarding address is not changed to the directly connected next-hop, but 0.0.0.0 is used in the type 5 LSA. Try it with a p2p link for example.

EdTheLad

PStefanov wrote: »

Thanks for the link. By the way, I encountered something I hadn't noticed before - if the external link (the link to a router that you receive an external route from) that runs OSPF is not an OSPF network type of either broadcast or non-broadcast, the forwarding address is not changed to the directly connected next-hop, but 0.0.0.0 is used in the type 5 LSA. Try it with a p2p link for example.

Yes,this feature is only available on multiaccess networks,it only really makes sense if 3 or more devces share a common network.