Please verify this OSPF Area Summary config

Unanswered Question
Oct 8th, 2009
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I was trying to use Dynamips to verify it, but Dynamips was running too slow on my computer. Can someone please verify if what I plan to do works?


Site A ----------- Site B


Both have WAN links into the cloud, running EIGRP with all other sites and wit each other. A and B also have a direct connection as backup link, running OSPF over the link.


Site A internal networks are in Area 20

Site B internal netowrks are in Area 30

Direct link between A and B is in Area 0


The direct link is used as a failover link to the WAN connections. I'm thinking to summary from Area 20 to Area 0 at A and from Area 30 to Area 0 at B as the fllowing:


Site A#

router ospf 1

area 20 range 10.0.0.0 255.255.255.0

area 20 range 192.168.0.0 255.255.0.0


Site B#

router ospf 1

area 20 range 10.0.0.0 255.255.255.0

area 20 range 192.168.0.0 255.255.0.0


10.0.0.0/8 and 192.168.0.0/16 can cover everything in the company. Is it ok that I have the same summary routes into Area 0 from two areas? My goal is to have either site learn the OSPF summary routes as from the direct link in between.


I think it works. I just don't have the tool to test right now.


Thanks a lot



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Giuseppe Larosa Thu, 10/08/2009 - 21:15
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Hello Gary,


>> 10.0.0.0/8 and 192.168.0.0/16 can cover everything in the company. Is it ok that I have the same summary routes into Area 0 from two areas?


No, this is not a good idea.

It can create routing problems.


Why a remote site should try to advertise a summary route for IP subnets that are not there?



I think in this case you need to create summary routes staying within area boundary.


Be also aware that the summary routes you create can hide details of routes in area 20 but if central site connects to other areas/remote sites the effect can be different from what you would expect.


You should be able with a good address plan to have remote site to advertise more specific per site summarized routes over the OSPF link.

Central site can advertise its own aggregated address and you can expect the summary routes coming from all other remote sites to be sent too.


This shouldn't be a problem because EIGRP is preferred over OSPF (lower AD).

Also if you don't use summarization in EIGRP you should have more specific routes.


Most specific routes are used first so if you send only a default in EIGRP for example the OSPF backup link will be used instead of primary EIGRP link for sending traffic from remote site to central site.


You need to balance between the desire to have as few prefixes as possible with the need to keep a hierarchy in using links where different routing protocols are running.


Hope to help

Giuseppe


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