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New Member

OSPF equal cost path

Hi all,

I have

ospf.jpg           

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.

1 ACCEPTED SOLUTION

Accepted Solutions

Re: OSPF equal cost path

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

11 REPLIES
Hall of Fame Super Blue

OSPF equal cost path

Fabio

Are C and D not seeing equal cost paths ?

Jon

New Member

OSPF equal cost path

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

Hall of Fame Super Blue

OSPF equal cost path

Fabio

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

Jon

New Member

OSPF equal cost path

Hi Jon,

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

Re: OSPF equal cost path

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

Silver

OSPF equal cost path

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.

Daniel Dib CCIE #37149 Please rate helpful posts.
New Member

OSPF equal cost path

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.

Re: OSPF equal cost path

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

New Member

Re: OSPF equal cost path

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

Thanks all.

Cheers,

Fabio.

New Member

Re: OSPF equal cost path

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!!!

Hall of Fame Super Blue

OSPF equal cost path

Rolf

Always welcome

Jon

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