Tie breaker for two identical ospf external type 2 routes

[Tatsuya]

Hi everyone,

I am testing ospf redistribution and would like to confirm if my understanding is correct regarding tie breaker for identical OSPF external type 2 routes.

I GNSed the following network:

-All the interfaces of Backbone router participate OSPF area 0

-Both ASBR1 and ASBR2 participate OSPF area 0 and redistribute static subnet 10.2.2.0/24

-Metric configuration is default setting

If I do not configure ospf interface cost on the Backbone router, it sees equal cost path on its routing table.

To make one path as the primary path, I set higher cost on the interface of the Backbone router which connects to ASBR2 issuing the following command:

ip ospf cost 60000

This successfully made the path via ASBR1 as the primary path.

I have read RFC2328 OSPF version 2 and found the following line:

Type 2 external paths advertising equal type 2 metrics are compared by looking at the distance to the forwarding addresses.

The distance to the forwarding addresses above also includes the interface cost manually configured on the interface, correct?

I will have to work on adding a backup router in a future project but the backbone router will be Meraki L3 switch.

If I want to replicate the above GNS3 network, I think I will need to set a higher cost on the interface of Meraki L3 switch which connects to a new back up router.

Regards,

Tats

[MHM Cisco World · Accepted solution]

I dont know why you confuse 
in ASBR you can config the metric the ASBR add to prefix it redistribute into OSPF. 
this will make you control which ASBR will be primary and which will backup. 

redistribute protocol [process-id] {level-1 | level-1-2 | level-2} [metric metric-value]
[metric-type type-value]

if you not change the default metric then the Link toward the ASBR will be the tie breaker. 

[Tatsuya]

Thanks!

That is another option I was thinking of but seems like changing seed metric on ASBR would be a better choice than configuration interface cost.

[Giuseppe Larosa · Accepted solution]

Hello @Tatsuya ,

>>

Type 2 external paths advertising equal type 2 metrics are compared by looking at the distance to the forwarding addresses.

The forwarding addresses in most cases has to be intended as the specific ASBR's OSPF RID.

This can be checked using

show ip ospf database external 10.2.2.0

To be noted if you see Forwarding address = 0.0.0.0 this means FA = ASBR OSPF RID if there is a value it can be the IP next hop of the static route redistributed into OSPF

The RFC says that in case of a tie of the O E2 external metric or seed metric the OSPF router compares the internal metric to the ASBRs or to be more general the metric to the forwarding address field listed in the previuos output.

To get a primary / secondary path for external prefix it would be better to increase the seed metric on ASBR2 instead of changing OSPF costs on links as this action impacts all types of routes either external or internal.

Also you could consider to accept equal cost multipaths if there are no Firewalls in the middle there is no issue on using ECMP

Final note about Forwarding Address:

the Forwarding Address when set to a value different then 0.0.0.0 has to be known in OSPF via an internal route or  the external LSA using the FA cannot be installed in IP routing table.

Hope to help

Giuseppe

[Tatsuya · Accepted solution]

Thanks for the explanation.

I will test redistribution with higher metric on ASBR2.

As for equal cost paths, the thing is that the other sites are some cloud providers and Meraki MX via S2SVPN so I believe there would be asymmetric routing occurred if I configured it on HQ device.

Regards,

Tatsuya