Route Summarization

Unanswered Question
Istvan_Rabai Sat, 02/16/2008 - 19:59

Hi Edward,

I understand from your post that you can calculate the summary-address and mask.

Once you have calculated them, the actual command you use is specific to routing protocols:

For EIGRP you summarize routes on interface level with the following command:

ip summary-address eigrp (as_number address mask)

For RIP, on interface level:

ip summary-address rip (address mask)

For OSPF ABR under the OSPF process:

area x range (address mask)

For OSPF ASBR under the OSPF process:

summary-address (address mask)

For BGP under the BGP process:

aggregate-address (address mask) [summary-only] [as-set]

The routing table itself will not be summarized on the router where you apply the summarization command, and the already existing routes will remain there.

The summarized route will be propagated by updates to other routers belonging to the same routing domain, e.g. to other EIGRP, or OSPF, or RIP etc. routers within the domain respectively. The summarized route will be put here into the routing table and will point to the interface leading toward the router that originated the summarized route.

So the routing table will be reduced (summarized) on the other routers within the EIGRP (OSPF, RIP, etc) routing domain, not on the router where you perform summarization.

On the local router, where you apply the summarizing command, the summarized route will appear in the routing table in addition to the already existing routes, but the summarized route will point to the null0 interface.

Does this answer your question?



dongdongliu Sun, 02/17/2008 - 02:26

Hi Edward

if you want to summarize several route entry to reduce route table size, first, to find which router send you these route, then, login that router and using summary command, after that, you just receive a summary route.



shivlu jain Sun, 02/17/2008 - 04:13

simply first check that no other route was advertised apart fro that area.

2nd step is to identify the ABR routers.

3rd router is to advertis the summary route.

Precaution:- Every abr in that network should be directly connected to the Backbone area.Summay address will be added on to the every ABR in the network. Else route leaking problem could occur.



lamav Sun, 02/17/2008 - 09:20

Istvan, that was an awesome post! Very informative....great.

Quick question...

In OSPF, how does the router know which interfaces to advertise those summary routes out of?

In EIGRP, its obvious, since the summary address is applied to the interface.

Can you refresh my memory....?

Istvan_Rabai Sun, 02/17/2008 - 10:29

Hi Victor,

In OSPF the concept is to summarize on area boundaries rather than on interfaces.

An OSPF ABR residing on the area boundary has separate OSPF databases for each area. The ABR will put the summarized routes into the OSPF database for areas other than the area from which the summarization was made.

For example: you want to summarize routes in area 1 to send only summary routes into the backbone (area 0), on the ABR connecting these 2 areas.

If you do summarization on routes present in the OSPF database for area 1 using the "area 1 range ...." command, then the summarized routes will be put into the OSPF database for area 0 on the ABR (the local router)

As new type-3 LSAs are created, these LSAs are propagated to other routers within area 0, so OSPF databases are synchronized within area 0. These routes are propagated as type-3 LSAs to other leaf areas as well through the respective ABRs.



Mohamed Sobair Sun, 02/17/2008 - 10:44


Existing routes in your routing table cant be summarized, but rather filtered.

If you want 2 summarize the existing routes, asks who is sending you those route to summarize them, or you filter them locally.



lamav Sun, 02/17/2008 - 11:03

Istvan, once again, awesome explanation. Its like reading a Cisco book -- only you make sense. lol

So, I guess it ssafe to say that if I have a router that has interfaces on, say, 3 separate areas (hypothetically), the summarized routes for the networks in one of those areas (area 1, lets say), will be sent out all other interfaces, except for the one that is connected to area 1, of course.

Is that about accurate?

Istvan_Rabai Sun, 02/17/2008 - 20:18

Hi Victor,

Yes, that's correct.

There is one exception, if you configure one or more areas as totally stubby area with this command as example:

"area 2 stub no-summary"

This will prevent summary LSAs from being injected into that area and allow only a default route.

You should once construct such a configuration and investigate the OSPF database for yourself to become very certain about what is happening in OSPF.

Thank you for your high opinion about my explanation :)



shivlu jain Mon, 02/18/2008 - 05:02

Even with this you can run one more command which is not covered in the ios.MAy be it helped you lot

sh ip ospf e




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