OSPF equal cost path

[fabio.marino]

Hi all,

I have

           

The network 192.168.0.0 is directly connected to Router A and B. Router A and B are redistributing the network in the ospf area 0.

Router A,B,C,D belong to the same OSPF Area.

Router C is DR. The cost of the link is indicated in the draw.

Question:

Router D (and router C) is using as next hop the Router A for reaching the network 192.168.0.0.

Why the router D is not using the router B as next hop due to the fact that all the possible paths are equal cost?

Thanks.

[Jon Marshall]

Fabio

Are C and D not seeing equal cost paths ?

Jon

[fabio.marino]

Every interface of every router inside the mentioned ospf process is seeing the cost ,emtioned in the draw (1)

[Jon Marshall]

Fabio

So do C and D have two equal cost paths in the IP routing table to that network ?

Jon

[fabio.marino]

Hi Jon,

no. Router B and C have only one entry in the routing table and both are using as next hop router A.

[Rolf Fischer]

Fabio,

Jon,

I hope you allow me to join.

Could you please post the output of

- show ip ospf border-routers

- show ip ospf database external 192.168.0.0

from Router D?

Points to focus on:

- metric-type E1/E2

- seed-metric (E2) / external metric (E1)

- costs to ASBRs A and B

- non-zero Forwarding Addresses in Type-5 LSAs

Regards

Rolf

[daniel.dib]

Can you paste the output of show ip protocols and show ip ospf database router x.x.x.x where x.x.x.x is the RID of Router A and B, C and D.

Daniel Dib
CCIE #37149

Please rate helpful posts.

[fabio.marino]

Hi All,

I got all your point. No output possible.

I will analise th answer of Rolf.

I just need an indication of what can be.

[Rolf Fischer · Accepted solution]

Fabio,

if the Forwarding Address is not 0.0.0.0, the next hop will be calculated by evaluating the OSPF internal path to the FA.

Useful link: The Effects of the Forwarding Address on Type 5 LSA Path Selection

Otherwise you'll have to look for the components of the costs: Cost to ASBR + Seed cost (E2; default: 20) or external cost (E1).

HTH

Rolf

[fabio.marino]

Let me check, I am really curiosi to figure out this point and of course I will let you know.

Thanks all.

Cheers,

Fabio.

[fabio.marino]

Hi,

At the end the information provided by Rolf fit in my case.

Playing with the cost to reach the ASBR and the metric of the external route I was able to figure out why the router A is the50

preferred exit point: during the redistribution router A is modifying the metric value, in this way C and D are using A as

Thanks all!!!

[Jon Marshall]

Rolf

Always welcome

Jon