More OSPF Filtering

I just finished watching Internetworkexpert Advanced video on OSPF LSA Filtering. In my last post I mentioned we can filter LSA type 3's with stub areas via the stub-no summary. The problem with this, is we will filter all LSA Type 3's.

We can also manually filter out LSA Type 3's. This is a cool option because we can be specific on what routes we want to filter to keep them from getting into the link state database, instead of filtering all Type 3's like stub area's. To do this, we create a prefix list to deny the prefix we want to filter i.e

ip prefix-list NOTME deny 150.1.5.0/24
ip prefix-list 0.0.0.0/0 le 32 (permits all other prefix, never forget the implicit deny!!!)

Then in our routing process we can call that prefix list and filter it either inbound or outbound.

** Note. You can filter based on a distribute list, however, the distribute list will not keep the prefix from going into the link-state database.