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New Member

OSPF route selection for external routes

Hi,

we have a Router A in a subarea. The router A has two connection to area 0 via different ABRs ( B and C). Both ABR are ASBR at the same time. In addition we have a third ASBR (D) in the area 0. ASBR C and D advertise the same external route (type 1) in the area 0. The ospf costs from A to C are 1000 and from A to D are 200.

The problem is that the Router A selects the route learned from ABR/ASBR C, although the path to ABR/ASBR C has higher costs.

Anybody knows the reason for this ?

5 REPLIES
New Member

Re: OSPF route selection for external routes

The reason is in rfc2328 16.4.1,the folloing is copy from rfc2328,it's the answer.

The path preference rules, stated from highest to lowest preference,are as follows.........

o Intra-area paths using non-backbone areas are always the most preferred.

o The other paths, intra-area backbone paths and inter-area paths, are of equal preference.

From A to C is a intra-area path in non-backbone,but From A to D is a inter-area path.

New Member

Re: OSPF route selection for external routes

What can be done about this behaviour?

I have a similar case. I this case an External type 2 path must be preferred over an Intra-area path. How can this be done??

Thanks in advance.

Harry

Cisco Employee

Re: OSPF route selection for external routes

There is no way this can be done for the same prefix with the same prefix lenght. What you could do though is injecting more specific prefixes as E2 than the ones advertised as Intra Area.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: OSPF route selection for external routes

Thanks.

But how can I inject more specific routes within OSPF?

Thanks in advance.

Kind regards.

Harry

Cisco Employee

Re: OSPF route selection for external routes

There is no feature as such to help you do that. If you can manage to originate the more specific route from the other protocol then you'd be fine. Can you tell me more about your specific scenario.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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