Filter lsa 5 in OSPF

[christof32]

Hello community,

An area 0 is partioned using an area 5 as a transit area.

The one side of area 0 contains  E2 routes that I want to filter so that they do not enter either area 5 or the other side of area 0.

One way that I think of is using area 5 as a stub area.

The only routes that I want to be advertised in the backbone (left and right)  are the ones injected in area 5 and it seems that it is working.

I want to ask how valid is this setup, using a stub area for a transit area between two backbone areas and if I should expect any problems.

Best Regards,

[Peter Paluch]

Hello Christof,

Your setup is not valid, unfortunately. If a backbone area is partitioned like yours, it must be made connected either using a virtual link or a tunnel. If you keep your network in the current state, without a virtual link or a tunnel, you can not, for example, enlarge the backbone area at either side with another area - for example, consider the sequence of areas 1-0-5-0. OSPF rules mandate that on ABRs, LSA-3 are processed only if received from a backbone area. This will cause the networks from area 1 to be unreachable in the rightmost area 0. In similar way, there could be other reachability issues occuring in your setup.

The kind of route filtering you are seeking for is hard to achieve with OSPF. The idea with having the area 5 configured as stub area is quite clever but you can not create a virtual link over a stub area - that is a deliberate OSPF limitation. Personally, I would strongly suggest creating a GRE or IP-in-IP tunnel over the area 5 between the ABRs, and running the OSPF over this tunnel (keeping this tunnel in area 0).

Best regards,

Peter

[Marwan ALshawi]

Peter what about using OSPF Transit Capability

http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/

Sent from Cisco Technical Support iPhone App

[Marwan ALshawi]

Igonre my last post I misunderstand the question

Then in your case in the router that has the external route and considered as asbr and abr you use the below command to filter out this lsa type 5 route

Let's say the external route is 4.4.4.3.0/24

summary-address 4.4.3.0 255.255.255.0 no-advertise

Hope this help

[christof32]

Thank you all for your help.

The problem is that the asbr is not also the abr in my case. And if i do this in the asbr, the E2 routes will be filtered not only in area 5 but in the whole ospf domain at my left side which is something I do not want.