Inter Area vs IntraArea routing (Distribution area to Backbone)

Unanswered Question
Nov 21st, 2008


Please see the attached diagram. I have a scenario that require the routing illustrated in the attached diagram. A have to ABRs connecting the distribution area to the backbone and one subnet attached to one of the ABRs. Ineed to reroute the traffic toward a backbone network via the other ABR (see the red path). Although I changeed the OSP cost to put a higher cost for the direct link the more expensive path take precedence because the destination route is learned via a backbone link I believe.

I have this problem too.
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Joseph W. Doherty Fri, 11/21/2008 - 17:47

Could you describe what area or areas the link or links are between the two ABR routers shown in area 1?

You've noted cost of 500 on two of the OSPF interfaces, what are the costs of interfaces on the other connections between the three routers?

Network 1 is within area 0?

Network 1 and 2 selected via OSPF interfaces matching network statements and not static injection or alike?

cristip Mon, 11/24/2008 - 04:50

First router at the top is in Area 0

the two in the middle are in area 0 with the upstream interfaces, all the other are in area 1. I believe the solution will be to move the link between the two ABR in area 0

the OSPF area are created with network statements, each ethernet segment has it's own C class network. The OSPF settings are the default excepting the cost I change as depicted. so consider all the costs 1.

I will try my solution this morning and let you u guys know th results

Any other suggestion will be much appreciated

Thank you


Joseph W. Doherty Mon, 11/24/2008 - 05:10

If the two area 1 routers' cross link is within area 1, once the traffic gets to the ABR it will not use the cross link in area 1. Moving the link to area 0 should correct the problem. However, it's often good to have that cross link, if possile, also in area 1. If you have ports to burn, have one in each area. In not, you might be able to VLAN trunk the areas across the one cross link.

As to costing, Cisco routers often have a default auto costing based on bandwidth and assume maximum bandwidth is 100 Mbps. So links of 100 Mbps or more would cost 1. (This can be overridden per interface, or change the auto costing base to support gig or 10 gig or multi-10 gig.)

Giuseppe Larosa Sat, 11/22/2008 - 02:56

Hello Cristian,

if the subnet is attached only to ABR2 OSPF will use the path on ABR2 to reach the backbone whatever you try unless you make ABR2 an internal router in non-zero area.

you can

a) move the subnet under ABR1

b) use PBR to override OSPF decisions and send traffic from ABR2 to ABR1 but take care of return path too.

c) use a GRE tunnel between ABR1 and ABR2 but you may still need PBR.

Hope to help


Giuseppe Larosa Mon, 11/24/2008 - 06:04

Hello Cristian,

>> What about moving the ABR to ABR2 link in area 0 ?

it should work and is the simplest way to do it

in this case changing costs on ABR2-Core2 link on both sides can make traffic go via ABR1.

Hope to help


cristip Mon, 11/24/2008 - 09:04

My initial idea (to move the ABR1 to ABR2 in area 0) worked. Problem solved.

Thank you



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