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New Member

Quick OSPF question

OK, I have two L3 switches running OSPF. One is in area 0 and area 120 (ABR) and the other is in area 120 but also redistributes connected and static subnets (making it an ASBR?). The ABR is configured with area 120 nssa no-summary and the other with area 120 nssa.

What I want to do is stop the ABR from advertising NSSA external type 2 routes to the ASBR as basically I want this switch to just have a default route (it doesn't need to know anything else as it only has one physical route out).

Is this possible? I'm getting myself mixed up thinking about it.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Hi,

Hi,

Okay, I get the point. So, basically you want your ASBR to ignore the external routes redistributed by your ABR (which is also an ASBR but let's keep the distinction) while all other routers in the area should see them.

I believe that this is a use case for distribute lists on the ASBR - filter out the routes entering the routing table based on some additional criteria. Can you please check if the IOS on the ASBR supports the distribute-list route-map route-map-name in command in the OSPF process configuration? If it does then the solution should be relatively easy to implement. On the ABR, redistribute the routes into OSPF with a specific tag value, say, 123. The tag value can be specified in the redistribute command using the tag tag-value keyword. On the ASBR, use the following construct:

route-map FilterOSPF deny 10
 match tag 123
 match route-type nssa-external
!
route-map FilterOSPF permit 20
!
router ospf 1
 distribute-list route-map FilterOSPF in

In OSPF, you cannot prevent LSA7 from being flooded throughout the NSSA area or from being processed at an arbitrary router. What you can do is only to prevent routes described by these LSA7 from entering the routing table, and this is what is accomplished by this.

Feel free to use the tag value and the name of the route-map as you see fit. Avoid the tag value 0 as that is the default tag value.

Best regards,
Peter

5 REPLIES
Cisco Employee

Hi,What I want to do is stop

Hi,

What I want to do is stop the ABR from advertising NSSA external type 2 routes to the ASBR

I am somewhat confused by this statement. Your ABR is not doing any redistribution, is it? What kind of NSSA external type 2 routes is it advertising toward the ASBR, then? It should be the other internal switch in area 120 doing the redistribution that advertises NSSA external routes.

Can you clarify please?

Best regards,
Peter

New Member

Yes, I should have also

Yes, I should have also stated that the ABR also has connected subnets and static routes that are in turn redistributed in to OSPF. 

Cisco Employee

Hi,

Hi,

Okay, I get the point. So, basically you want your ASBR to ignore the external routes redistributed by your ABR (which is also an ASBR but let's keep the distinction) while all other routers in the area should see them.

I believe that this is a use case for distribute lists on the ASBR - filter out the routes entering the routing table based on some additional criteria. Can you please check if the IOS on the ASBR supports the distribute-list route-map route-map-name in command in the OSPF process configuration? If it does then the solution should be relatively easy to implement. On the ABR, redistribute the routes into OSPF with a specific tag value, say, 123. The tag value can be specified in the redistribute command using the tag tag-value keyword. On the ASBR, use the following construct:

route-map FilterOSPF deny 10
 match tag 123
 match route-type nssa-external
!
route-map FilterOSPF permit 20
!
router ospf 1
 distribute-list route-map FilterOSPF in

In OSPF, you cannot prevent LSA7 from being flooded throughout the NSSA area or from being processed at an arbitrary router. What you can do is only to prevent routes described by these LSA7 from entering the routing table, and this is what is accomplished by this.

Feel free to use the tag value and the name of the route-map as you see fit. Avoid the tag value 0 as that is the default tag value.

Best regards,
Peter

New Member

That sir, is most excellent.

That sir, is most excellent. Works a treat! Thanks a bunch. I know it's just a bit OCD but I like to keep my routing table as clean as possible.

Cisco Employee

Hi,I am very glad it worked

Hi,

I am very glad it worked for you :)

Best regards,
Peter

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