Default route cost change in stub area

[nuggetinu]

Hi,

First of all, I have the attached setup.

There is a static route from R3 to R4 which is being redistributed into OSPF. What I am noticing is that before setting area 51 to a stub area, the default route advertised to R1 has a cost of 1 [110/1].

Then, when setting area 51 on both R1 and R2 as stub, the cost on R1 changes to 65 [110/65].

Can someone explain the reason behind this? I understand that the Serial link between R1 and R2 is considered a T1 1.544Mbps link which translates to a cost of 64, but why is this initially cost ommitted pre configuring the area as stub?

Thanks

[Rolf Fischer · Accepted solution]

Hi,

most probably you used the default metric-type (E2) when you originated a default-route on a router in the regular area. You will see then an external OSPF default-route with the (seed)cost unchanged in your OSPF domain. When you change the metric type to E1 (default-information originate metric-type 1), the path cost will be added.

In a stub area, the (summary) default-route is generated by a ABR; and in this case it is an OSPF internal (IA - inter area) route. The cost of that route corresponds to the path to the ABR plus the StubDefaultCost (1 by default).

HTH
Rolf

[nuggetinu]

But I only saw the change after changing area 51 to stub. Removing this area from stub brings back the cost to [110/1]

[Jon Marshall]

Think that is what Rolf is saying. 

When you originate the default route from R3 it is will use the default metric type of E2 which does not use the cost(s) of the link(s) to the ASBR. 

When you configure the area as a stub area though R1 no longer receives the route from R3, instead R2 generates a new default and sends it to R1 and then the cost of the link between R1 and R2 is taken into account. 

Jon

[nuggetinu]

Ok now I understand! Thanks!