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Using area <> filter-llist command in ospf







This document provides a sample configuration on how to use area <> filter-list command. This feature was introduced in IOS release 12.0(15)S and is used for filtering LSA type 3 only. This filter can be used to filter type 3 LSA coming into an area, going out of an area or both.




It is assumed that a person reading this document is familiar with Ospf.


Components used


For this document 3700 Series router with IOS 12.4(18) loaded in it.


Network Diagram


Area filter- ospf.jpg



The configuration files are attached with this document.


For this example we will take network attached to loopback 1 ( and filter that out in a way that routers in Area 1 don't see it.


Before we start applying filters lets take a look at routing table of R2 and R3:


From Router R2:




And, we can surely see network Now, to make sure that we have it on Router R3 as well, lets have a look at its routng table:




and, its there as well.


Now for filtering network from area 1 we need to perform two tasks


  • We first need to configure a prefix-list which will filter this particular network out.
  • Second, we need to apply that filter


This will be done at router R2, which is the ABR for Area 1.


For step 1, we will create a prefix-list which will filter net and permit all others:




For step 2, we will apply this filter:


apply filter.jpg

We can also apply the same filter in out direction but then we would have to do it on Area 0 as this is the area from which this network is advertised out.



Lets see the routing table of Router R3:


R3 after filter.jpg


We surely don't see network on R3. And, to prove that R2 is not sending type 3 LSA for this network in area 1, lets see the database for network on R2:



R2 after filter.jpg

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