OSPF Forward Metric

Unanswered Question
Dec 12th, 2008

Hi,

I have set OSPF to choose between two paths. Here is the output from what I have:

lab_R1#sh ip route 10.254.7.1

Routing entry for 10.254.7.0/30

Known via "ospf 10", distance 110, metric 28, type intra area

Last update from 172.16.1.193 on Vlan1, 00:26:01 ago

Routing Descriptor Blocks:

* 172.16.1.193, from 172.16.1.17, 00:26:01 ago, via Vlan1

Route metric is 28, traffic share count is 1

lab_R1#sh ip route 10.254.6.73

Routing entry for 10.254.6.72/30

Known via "ospf 10", distance 110, metric 23, type intra area

Last update from 172.16.1.193 on Vlan1, 00:26:05 ago

Routing Descriptor Blocks:

* 172.16.1.193, from 172.16.1.19, 00:26:05 ago, via Vlan1

Route metric is 23, traffic share count is 1

as you can see the route to 10.254.7.1 has a higher cost, so it should no be the preferred link, but in fact this is the preferred link when I make a trace.

I also have notice in the end routers that I am receiving this routes, but the 10.254.7.1 has a lower "Forward Metric", I'm not sure if this is the reason for this behavior.

Also I tried confioguring the ip ospf X command, but it still prefer the 10.254.7.1 path, no matter what cost is on the other.

Any idea on the Forward Metric topic? I have being researching on the web but there is no clear information about it.

regards...

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Giuseppe Larosa Sat, 12/13/2008 - 04:31

Hello Seeker,

the two sh ip route you have posted are about two different disjointed /30 IP subnets:

Routing entry for 10.254.7.0/30

Routing entry for 10.254.6.72/30

Then if you have inter Area routes or external routes of type O E2 the forward metric can be the metric to reach the ASBR.

The O E2 type of external routes keep separated the seed metric and doesn't sum it to the internal part of the metric.

However, by running SPF over LSA type 4 (ASBR summary LSA) every router can calculate the forward metric to reach the LSA type 5 owner/originator.

So forward metric should refer to the metric to reach the ASBR/ABR and may differ from the next-hop used.

If the OSPF Router-IDs are advertised in OSPF I think that forward metric is the metric to reach ASBR/ABR router-id ip address if forwarding address=0.0.0.0 (=this node) in LSA type 5.

Or if the forwarding address in the LSA type 5 is different from 0.0.0.0 it is the metric to reach that ip address.

Hope to help

Giuseppe

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