OSPF Exterior Route Filtering not taking place

July 29, 2022

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There have been conditions whereby we see that in OSPF exterior routes usually are not filtered on ABR by any of the routers. So we’ve got to configure the exterior route Filtration on the ASBR Router solely. If the Routes enter OSPF area and traverses with out being filtered at ASBR, no filter-list command may be fruitful. Filter-list will solely filter Kind 3 LSA and never Kind 5 LSA. Solely workaround could be to configure distribute listing on the vacation spot OSPF router requiring restricted routes to be learnt.

Instance State of affairs: OSPF Exterior Route Filtering not taking place

Let’s perceive the identical with an instance –

Router R1 is studying Routes from inside OSPF area (192.168.x.x and Router Loopbacks) and Exterior Community area (172.16.x.x).Beneath snapshot is the output from “present ip route ospf” –

Now , on the ASBR Router we’ll redistribute solely community 172.16.4.0/24 community and deny 172.16.1.0/24 , 172.16.2.0/24 , 172.15.3.0/24 from being propagated into OSPF area.

Now that we configured selective redistribution (of 172.16.4.0/24 community solely) on ASBR , lets see the output on R1 is per expectation –

Subsequent , now we take away the selective redistribution from R4 and permit all of the 172 networks being marketed as under

Now, let’s see how the “space filter-list” command handles the Kind 5 and Kind 3 LSA.

Vital to notice that 172.16.1.0/24 as much as 172.16.4.0/24 are exterior LSA Typ5 learnt routes whereas 2.2.2.0/24, 3.3.3.0/24, 4.4.4.0/24 and 192.166.23.0/24, 192.168.34.0/24 are LSA Kind 3 learnt routes. Now on R2, we create Space filter listing to disclaim all of the inter-area and externally leant routes by way of under instructions –

As soon as the filter listing on R2 ABR is configured to filter routes learnt by Space 1 , the “present ip route ospf” exhibits that externally learnt routes are nonetheless displaying in routing desk of R1 whereas Inter-area (LSA Kind 3 learnt) routes are nonetheless filtered.