Can I have same network in 2 different OSPF areas

ud0508500 · ‎02-01-2009

I have a server network area 12 at one locaton and an area 132 at different locations that are sharing the same network 157.171.53.0 /24 via glpb..

I am getting

An ospfIfConfigError trap has been received for router 10.69.12.1. The interface is 157.171.53.3, and the error is AREA-MISMATCH

can I have the same network in two different areas using OSPF? Thanks again fro your help and time

scottmac · ‎02-01-2009

No, not in any reasonable configuration.

Different areas implies different network segments ... routers can't/won't route to the same network ... that's what a bridge would do.

You could probably do some things with NAT and make it work if you had to, but ultimately, you'd be better off just doing the re-addressing as soon as practical and possible.

paolo bevilacqua · ‎02-01-2009

But may be the desire is to have differently routed paths for redundancy to backbone, and due to OSPF topology, the problem.

So ultimately what you want is the net fall into one area only and someway connect the glbp routers to area 0.

Not even sure a virtual link would work as workaround.

Giuseppe Larosa · ‎02-02-2009

Hello Jay,

the error you see is caused because both routers see each other OSPF hellos.

to stop seeing these messages you need to passive the interface

router ospf 10

passive-interface

there is no need for them to exchange messages they cannot become neighbors for the AREA mismatch.

Be aware that OSPF implements a strict hierachical routing:

inside area 12 the return path will use the intra area routearea 12 in in ( O) regardless of metric

inside area 132 the return path will use the intra area route in area 132 (O).

From OSPF area 0 and other OSPF area the two O IA (inter area ) routes will be compared and the best will be used (nearest ABR and lowest metric in LSA type 3 as a sum).

You can at least face asymmetrical routing.

You should put the link in a single OSPF area present on both routers and connected to area 0.

An ABR can be ABR of multiple areas to area 0.

So the best fix here is to build a legitimate topology in a common area (may be a new one) that reach area 0.

Putting the same IP network in two different OSPF areas is acceptable only when you use two different OSPF processes (but it is a different scenario from yours)

Hope to help

Giuseppe