Route Filtering Question for OSPF

Answered Question
Aug 22nd, 2008
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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 11 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 11 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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Correct Answer
tdrais Fri, 08/22/2008 - 05:38
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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

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