OSPF summarization within an area !!

Unanswered Question
Feb 3rd, 2008

Hi, i have many sites that have only 1 active link to the hub at any time, now normally in an area i have approx 60+ routes on each router whereas the next hop for every route is same i.e. ABR of that area, so is there a way i can summarize all the routes ?? they may not be contiguous so is there any option that forces the ABR or internal router to keep certain summary routes instead of complete 60+ routes ?


I have this problem too.
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simonstoll Mon, 02/04/2008 - 00:06


As OSPF is a link state Protokoll, you cannot summarize within an area. All linkstate databases on all routers within an area must contain the same data. What you can do is using different areas for you sites and make them all stub or totaly stub. Another option is to use a differnt routing protocoll such as EIGRP where you can summarize anywhere you like.


shrikar.dange Mon, 02/04/2008 - 02:09


As simon says you can not summarise within a area but you can summarise the networks at area boundary or at ABR.

The area range command is used only with Area Border Routers (ABRs). It is used to summarize routes for an area. The result is that a single summary route is advertised to other areas by the ABR. External to the area, a single route is advertised for each address range.Multiple area router configuration commands specifying the range option can be configured. Thus, OSPF can summarize addresses for many different sets of address ranges.

The command is:

area area-id range ip-address ip-address-mask [advertise | not-advertise] [cost cost]


area 1 range advertise cost 10


area-id is the Identifier of the area about which routes are to be summarized.

In case of OSPF you can summarise only at 2 points in the network which are at ABR with above command and at ASBR using summary-address command.



shri :)

royalblues Mon, 02/04/2008 - 02:57

Not sure whether this will satisfy your needs but you can configure your spokes as totally stubby area which would mean the ABR would just send out only a default route and will block all external and summary routes from other areas

Just understand that within a given area all the routers will have identical link state database but the only difference would be the number of routes



illusion_rox Tue, 02/05/2008 - 21:58

hi, i know that the LS database must be same on all routers what i was seeking is that the database shall be identical but routing table should be smaller in size,

Anyway thanks for every1's feedback :)

simonstoll Wed, 02/06/2008 - 01:31


If the size of your routing table is the problem, here is the solution:

You can controll with a a distribute list in the OSPF Prozess what information gets out of the LSDB in the Routing Table. Here is an example to just import the default route:

router ospf 10

network area 0

distribute-list DEFAULT in


ip access-list standard DEFAULT


This distribute list has nothing to do with the LSDB Updates send to other neighbors, it just controlls the interaction of the LSDB to to the FIB.

Hope thats what you where looking for :-)



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