OSPF Forward Metric question

[Alexey Stytsenko]

Good day.

Please review topology used:

R1 and R3 - both ASBRs and redistributes information about connected networks 11.11.11.0/24 and 33.33.33.0/24. AREA1 is NSSA Area.

Both interfaces on R2 in directions to R1 and R3 have cost of 10. But take a look at R2 output:

R2#sh ip route 11.11.11.0
Routing entry for 11.11.11.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 10
  Last update from 192.168.12.1 on FastEthernet0/0, 00:00:06 ago
  Routing Descriptor Blocks:
  * 192.168.12.1, from 1.1.1.1, 00:00:06 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1

R2#sh ip route 33.33.33.0
Routing entry for 33.33.33.0/24
  Known via "ospf 1", distance 110, metric 20, type NSSA extern 2, forward metric 11
  Last update from 192.168.23.3 on FastEthernet0/1, 00:00:58 ago
  Routing Descriptor Blocks:
  * 192.168.23.3, from 3.3.3.3, 00:00:58 ago, via FastEthernet0/1
      Route metric is 20, traffic share count is 1

Forward metric to route from NSSA area is 11!

Lets check boundary routers output

R2#sh ip ospf border-routers

OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 1.1.1.1 [10] via 192.168.12.1, FastEthernet0/0, ASBR, Area 0, SPF 8
i 3.3.3.3 [10] via 192.168.23.3, FastEthernet0/1, ASBR, Area 1, SPF 6

Cost to both ASBRs same.  But why is forward metric different? Obviously this is somehow connected to NSSA but i cant find place where this is described. Can somebody point me where I can find description of this behaviour? At RFC 3101 I found nothing about this.

[Alexey Stytsenko]

You didnt get my point - i mean cos i redistributed connected  loopback interface, therefore its own cost cant affect forward metric calcultions (ospf even not enabled on it).

BTW i checked changing cost on Lo0 (3.3.3.3) and it seems that  its cost affects  forward metric of 33.33.33.0/24. So despite route is not "static with Lo next hop", Lo0 cost anyway affects Forward Metric, obviously because it listed as the forward address.

The question remaining is why are the Enrico`s results are different in the same conditions ^^

[Rolf Fischer]

Ah, now I understand :)

Well, I guess Enrico's setup was slightly different. You should see the same result as he did e.g. when you remove the 3.3.3.3 network statement under R3's OSPF process.

Thanks for using the rating system!

[Alexey Stytsenko]

You are absolutely right, tnx!