Route Filtering Question for OSPF

Answered Question
Aug 22nd, 2008

Hi,

Here's the situation …. I have a remote router acting as an ABR for both OSPF area 5 and area 0. In area 5, I have a network address of 10.1.2.0 /24 that I want to keep advertising in area 5 but not to have the network advertised via area 0. The reason for this is the network is a conflict that can't be removed at this time.

What's easier, to try to filter on this specific router or to filter on the far end router, that is a backbone router. Any suggestions? i.e route map vs. distribute list vs. pre-fix list?? I would appreciate a sample config as to how this could be done.

Thanks,

Steve

Correct Answer by Giuseppe Larosa about 8 years 6 months ago

Hello Steve,

have the net 10.1.2.0/24 advertised in area 5.

At the ABR(5,0) you can filter it by using an area filter-list.

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1011184

or area 5 range 10.1.2.0 255.255.255.0 not-advertise under router ospf process on ABR(0,5).

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1011537

Avoid to have this subnet redistributed into OSPF because LSA type 5 cannot be filtered out of the OSPF database.

Hope to help

Giuseppe

Correct Answer by tdrais about 8 years 6 months ago

All will depends on the type of route this network appears as and the types of areas you are using.

In the best case if it appears as a type 3 . Then you can use prefix lists to filter on area. When the routes appear as externals or intra area routes your options are very limited.

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t11/feature/guide/ft11at3f.html

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stephenshaw Fri, 08/22/2008 - 06:00

Thanks, interesting way to handle this. The newtork actually originates on a VLAN on the ABR (it's a L3 switch) so based on the link, I should be able to filter in area 5 outbound to prevent area 0 from seeing the network.

Appreciate the quick response.

Cheers,

Steve

Correct Answer
Giuseppe Larosa Fri, 08/22/2008 - 06:00

Hello Steve,

have the net 10.1.2.0/24 advertised in area 5.

At the ABR(5,0) you can filter it by using an area filter-list.

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1011184

or area 5 range 10.1.2.0 255.255.255.0 not-advertise under router ospf process on ABR(0,5).

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1011537

Avoid to have this subnet redistributed into OSPF because LSA type 5 cannot be filtered out of the OSPF database.

Hope to help

Giuseppe

stephenshaw Fri, 08/22/2008 - 09:27

Hi,

I did a lab test and it worked perfect - exactly as I need.

Thanks to both of you for your help.

Steve

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