OSPF metrics decision when redisributing eigrp

[nuggetinu]

Hi I am doing lab http://www.smartpctricks.com/2014/04/redistribution-between-eigrp-and-ospf.html

and am finding it difficult understanding the part where redistribution of EIGRP is occuring into OSPF.

There's the command:

router ospf 1

redistribute eigrp 1 metric 10 subnets

Can someone explain how it came to the conclusion that the metric should be "10"?

Thanks in advance.

[Predrag Jovic]

It is just seed metric when route is redistributed into OSPF. It is relevant only for calculations inside OSPF process to calculate best path to external network. You can assign to seed value whatever you believe it is "good start metric".

[nuggetinu]

Is this also the same when redistributing OSPF into EIGRP and come up with values like: redistribute ospf 1 metric 1500 1000 255 1

Is this also a seed metric (whatever you believe it is "good start metric")?

[Predrag Jovic]

Yes it is seed metric too. Seed metric is initial metric that route is getting during redistribution into routing protocol. For EIGRP and RIP default seed metric if not specified is infinite, so some seed metric need to be specified.

[Dennis Mink]

that 10 i believe is the ospf cost for ethernet on your N6 link

[nuggetinu]

Shouldn’t that cost be added to the serial link between R1 and R2 in that case with final cost being 64+10? If not, why?

[Predrag Jovic]

Not really, you can assign any metric you want, it does not have to be related to anything. Also, default external type is E2 and there is no adding cost to E2 type of external routes (inside of OSPF AS).

[nuggetinu]

So in that case, it should have been a cost of 64 if it is referring to the N6 link right? But in redistribution is it doesn't have to be precise as it's just have to be a good start metric.

[Predrag Jovic · Accepted solution]

Not really. You can set any seed metric value that you want as long as it will match some of your criteria.

For example. If you have two exit points from your AS to the same networks you redistribute with different seed metric to make one link preferred to other link (there are other ways to do it, but it is out of scope here). Since seed metric is relevant for OSPF's best path calculation, it does not really matter what is metric assigned to some other protocol, or interface, or whatever...

To try to explain it a little bit different. If there is no metric configured in redistribution command (when redistributing from other protocols) default seed metric is 20 (except for the case that route is redistributed from BGP - in which case it would be one). Value of seed metric, generally, can be important only in the cases where there is more than one copy of the same route.

[nuggetinu]

Thanks for the continuous feedback! Really appreciated