OSPF Filtering

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Jun 25th, 2007
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we have a problem to filter ospf routes in our network. It doesn?t work with ACLs.

Then we followed the description of the document ?IOS Release 12.2T OSPF ABR Type 3 LSA Filtering?. Problematic is that the affected switch is a cisco catalyst 3750. This switch is not listed under the supported devices of the above-mentioned documentation. Nevertheless he ?knows? the commands. It doesn?t work (should it???).

What other possibilities are there for filtering ospf routes (concerning this type of switch)?

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royalblues Mon, 06/25/2007 - 05:43
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Can you post your topology and let us know what exactly you want to achieve


Richard Burts Mon, 06/25/2007 - 06:34
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As you are discovering there are not many options for filtering OSPF. This is in part because the design of the protocol as a link state protocol is based on the assumption that the routers need a complete understanding of the network topology so that they can draw an accurate topology map and avoid loops.

There is limited ability to use access lists with distribute lists. Distribute list outbound for OSPF is not supported at all. Inbound distribute list for OSPF is supported but does not operate as it does for other routing protocols or as some people assume. If you configure an inbound distribute list you can prevent some routes from showing up in the routing table. But the LSAs (which is how the routes are advertised) are still in the Link State Data Base and will still be advertised to other OSPF neighbors. So using distribute list inbound is only locally effective and does not affect what is advertised to other neighbors.



mohammedmahmoud Mon, 06/25/2007 - 10:46
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Totally agree with Rick, but just to add a small detail, you can also do filtering Between OSPF Areas - LSA type 3 filtering on ABRs - The ABRs can filter network addresses being advertised by type 3 LSAs either into or out of an area.

Example (from Jeff Doyle's)

router ospf 1

area 25 filter-list prefix area25outbound out


ip prefix-list area25outbound seq 10 deny

ip prefix-list area25outbound seq 20 permit le 32

I've checked the feature navigator for you, and it seems that the 3750 doesn't support OSPF ABR type 3 LSA Filtering .


Mohammed Mahmoud.

farkascsgy Mon, 06/25/2007 - 06:19
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Distribuiton-list, route-map are there...



Please rate me if I helped.


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