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New Member

design a ospf network

WAN: Two 7505 connect to 30 or more branchs(3660)

LAN: 8540 5500(rsm)

there are several routers in each branch(connect to

branch 3660).

Now I want to use OSPF.

It is said that I can not place more than 50 routers

into one area, and can not place too many area in

one router(ABR).

If I set LAN and two 7505 (F0/0) into area0,

7505 will contain too many areas (each branch in one


If I set 7505 and all branchs 3660 into area 0,

I think there too many routers in area 0(it will

grow to more than 60 branches), and area 0 will be affected by unstable WAN connection.

Can anybody tell me how to design the ospf area?


New Member

Re: design a ospf network

I cannot understand your topology, but what is always recommended is that you design OSPF with the three level heirarchy in mind.

For example, you can put the core routers in area 0, connect your external networks to your core through ditribution routers, in which case, your boundary routers cab be either on the distribution/access boundary, which is recommended.

Thus , your HQ lan will connect directly to your core through a ABR, while your external networks(remote sites) can connect to your core through a distribution layer routers...which means those ditribution layer routers can be configured as boundary routers.

Its always a good idea not to put your ABR's or ASBR's on the core routers, as things like summarizations should not be performed on CORE.

Of what I get from your description, I think the best scalable solution is to buy some more routers for your distribution layer. They dont have to be really high end.

New Member

Re: design a ospf network

You don't give enough detail in your scenario. There are many issues here. 50 routers in an area is a rule of thumb, not a limitation. It all depends on how you have your networked designed.

If your branch offices have no ASBRs and you're using a hub and spoke model, each branch could be a totally stubby area. Also, you could group branches together into logical areas to cut down on the size of having multiple topological databases on the ABRs.

How many routers at each branch are there?

How many networks at each branch are there?

What is the current CPU/Memory utilization of your 7500s? 5500 RSM?

Do the branches interconnect (without the 7500s)?

Is there a comms backup strategy?

What is the speed and type of the primary connection?


New Member

Re: design a ospf network

Unless there is some compelling reason to use OSPF, you should use EIGRP. 50 or so branches does not warrant OSPF. You'll find that network convergence is a hell of a lot faster with EIGRP.

OSPF is designed for much larger networks and for distributed route architectures. It seems like everything is coming back to one or two routers.