Few days back I was called up for a problem of area summary of particular region. Guys did the summary with the proper command on routers but they were unable to receive the summary prefix. After that I checked the routers and seem everything on right place but why the summary route was not originating towards the area 0. After that I asked them whether any other router in area 100 is having any connectivity with area 0. They replied me by saying one connectivity was there but that serial interface link was put in passive mode. It means only two direct connectivity with area 0.
The next question I asked them to show the diagram. From the diagram which is depicted as in Figure 1, I was aghast to see there were three ABRs but actually summary command was added only on PE1 & PE2. I asked a question whether the summary command was added on PE3 or not. I got no as answer which actually I was expecting from them. So the culprit was PE3 and as soon as the command was added, a summary route was received in area 0.
Then I was questioned why we need to give the summary command on PE3 also still don't have any direct connectivity with area 0? I replied, “You can do the summary only on the ABR (Area Border Routers)”. As per the figure you were having 3 ABRs so we need to do summary on all the ABRs which were coming in area 100.
Note: - Area summarization will always be done on ABRs which are present in that area whether they are having any direct or indirect connectivity with area 0. ABR is that router which is having one leg in area 0 whether that is direct or indirect leg.