There have been situations wherein we see that in ospf external routes are not able to summarize by any of the ABR routers. So we have to configure the external route summarization on the ASBR Router only. If the Routes enter OSPF domain and traverses without being summarized at ASBR, no range or summary-address command can be fruitful. Let’s understand the same with an example –
First look at the behavior without using the command –
Router R1 is learning Routes from internal OSPF domain (192.168.x.x and Router Loopbacks) and External Network domain (172.16.x.x).Below snapshot is the output from “show ip route ospf” –
Now, on the R4 Router which is ASBR, we summarize external routes (Type 5) via “Summary-address ….” Command as shown below –
Now lets see the change in OSPF learnt routes on Router R1 –
That’s good, however what happens when we try to summarize External OSPF routes on Internal ABR Routers.
Before that, we remove the “summary-address” from R4 as below –
The output on R1 post removing summarization from R4 is –
Now , we configure summary-address on ABR Router R3 –
Alas, no summary. Hence from this testing lab, we clearly understand that summarization of Type 5 LSA (External Routes) can only happen on ASBR. Once the external LSA enters the OSPF domain, no Internal/ABR router can summarize it. “Summary-address” or “Area x Range” commands will not give the required objective.
Link for Inter Area Route summarization: