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OSPF Summarisation

We have recently changed our core network. Previously 3800 routers were used as ABRs on the core providing E1 connectivity to remote offices. OSPF Summarisation was used and a Summary address was created for each ABR on the core at each core site. 2 routers were installed at each core site to eliminate any single points of failure.

BEFORE.jpg

Now that ethernet circuits are more widely available, we have found it cheaper to move to a L3 switched core which now provide the ABR function. As the WAN circuits were moved from the existing routers to the L3 switches we had to remove the summarisation address for each router as the subnets were now used across multiple devices.

AFTER.jpg

My question is, can we create a summary address 10.1.0.0/15 at each layer 3 to advertise this route only to the rest of the core network at each L3 switch?

If so, is this just a case of issuing the command: area 1 range 10.1.0.0 255.254.0.0 at each layer 3 switch?

If not, what is the best way to solve this problem without having to change IP addressing at each site.

Thanks in advance

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Cisco Employee

Re: OSPF Summarisation

In the new topology it looks like you have a single OSPF Area (1) behind the L3 switches. In this case you can originate a /15 summary from each ABR towards the network core assuming your IP Addressing allows you to do that (proper hierarchy). I would however add a link between the two L3 switches in Area 1 as some of your branches are single homed and in absence of such a link return traffic (core to branch) can potentially take sub-optimal path by traversing another branch to reach the single homed branch. The Area 1 link between ABRs will also help in failure scenarios for dual homed branches to prevent the same problem of traffic traversing other branches to reach the branch with a failed uplink.

That being said if this network is expected to grow you can probably add scalability and modularity to it but that is a separate discussion.

Atif

2 REPLIES
Cisco Employee

Re: OSPF Summarisation

In the new topology it looks like you have a single OSPF Area (1) behind the L3 switches. In this case you can originate a /15 summary from each ABR towards the network core assuming your IP Addressing allows you to do that (proper hierarchy). I would however add a link between the two L3 switches in Area 1 as some of your branches are single homed and in absence of such a link return traffic (core to branch) can potentially take sub-optimal path by traversing another branch to reach the single homed branch. The Area 1 link between ABRs will also help in failure scenarios for dual homed branches to prevent the same problem of traffic traversing other branches to reach the branch with a failed uplink.

That being said if this network is expected to grow you can probably add scalability and modularity to it but that is a separate discussion.

Atif

New Member

Re: OSPF Summarisation

Will give this a go. Thanks for your help

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