OSPF Exterior Route Summarization not occurring

July 28, 2022

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There have been conditions whereby we see that in OSPF exterior routes are usually not in a position to summarize by any of the ABR routers. So we’ve got to configure the exterior route summarization on the ASBR Router solely. If the Routes enter OSPF area and traverses with out being summarized at ASBR, no vary or summary-address command may be fruitful.

Instance State of affairs: OSPF Exterior Route Summarization not occurring

Let’s perceive the identical with an instance. First take a look at the habits with out utilizing the command –

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

Now, on the R4 Router which is ASBR, we summarize exterior routes (Kind 5) through “Abstract-address ….” Command as proven beneath –

Now lets see the change in OSPF learnt routes on Router R1 –

That’s good, nonetheless what occurs after we attempt to summarize Exterior OSPF routes on Inner ABR Routers.

Earlier than that, we take away the “summary-address” from R4 as beneath –

The output on R1 publish eradicating summarization from R4 is –

Now , we configure summary-address on ABR Router R3 –

Alas, no abstract. Therefore from this testing lab, we clearly perceive that summarization of Kind 5 LSA (Exterior Routes) can solely occur on ASBR. As soon as the exterior LSA enters the OSPF area, no Inner/ABR router can summarize it. “Abstract-address” or “Space x Vary” instructions won’t give the required goal.