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New Member

Dual 3845 branch edge with OSPF

Hello,

We are considering a dual router branch office design and wanted to hear from anyone who has done this already, and what their general experience was (eg. “went great but watch out for…”).

We have dual 6509-Es in our HQ anchoring T1s and T3s for ~20 branches. Our largest branch currently has a single 3845 anchoring dual T3s (one to each 6509) and 6 multilinked T1s (for backup).

We have a business requirement to make this office “highly redundant” and one of the only single-points-of-failure left is the 3845.

Our thought is to add a second 3845, split the T3s between them, and cross connect them with two branch 4510s acting as backbones. Redundancy achieved through L3 OSPF routing, not STP, HSRP, etc., allowing us to use all of our bandwidth with quick failover.

Also…

* We have all devices in Area 0. Yes we know that's not ideal.

* We use Cisco phones at the branches connecting to call managers in HQ.

* 3 voice PRIs would be split across both routers.

* Internal switches are all L3 routing OSPF with connections to both cores.

I saw some useful information in both the High Availability Campus Network Design and the Enterprise Branch Office Design, but nothing beats what you can learn from the experience of others.

Any suggestions or comments based on the experience of others?

Thanks!

Mark

1 ACCEPTED SOLUTION

Accepted Solutions
Super Bronze

Re: Dual 3845 branch edge with OSPF

Using /30s works fine, or we've found letting the WAN edge peer with the LAN on a dedicated subnet works well too. The latter saves one less subnet per occurance within the topology.

If you have dual LAN routers, also best, if possible, to have dual links from each WAN router. That way you don't lose half your WAN bandwidth with loss of a LAN router.

No, don't believe GLBP has an advantage over OSPF if you have a LAN router in front of the WAN routers and if you have equal cost paths. If you need to support unequal cost paths, believe then GLBP could help. Or, again, in situations where you don't have an internal LAN router to provide ECMP.

With regard to remote trouble shooting, you can also wire one router's aux port to the other's console port and use reverse telnet.

With multiple paths, you can also perform IOS reloads during business hours. (Best to cost the links such that the target router stop forwarding traffic before doing the reload.)

4 REPLIES
Super Bronze

Re: Dual 3845 branch edge with OSPF

Keep in mind with the addition of second branch routers, you're going to increase the size of the OSPF topology. You might need to consider implementation of areas where there hasn't been a need to date.

Load balancing with OSPF was a chore at branches where the WAN router was also the branch's LAN router. GLBP is a huge aid.

Although you eliminate the branch WAN router being a single point of failure, users will have to understand lost of one router degrades performance.

When one router is having a problem, often you can support it inband via the second router path. (I.e. if you have dial-up lines on existing routers, you might discontinue them.)

New Member

Re: Dual 3845 branch edge with OSPF

Thanks for the reply.

In our environment we're hoping to keep it simple by using /30 connections between the backbone 4510s and the edge 3845s and letting OSPF populate ECMP routes for the 4510s. We're ok with load balancing being a little imperfect.

Would there be an advantage to going GLBP over that?

To your last point, yes that's one of the big advantages we're thinking of. We do a lot of remote troubleshooting. :)

Thanks again,

Mark

Super Bronze

Re: Dual 3845 branch edge with OSPF

Using /30s works fine, or we've found letting the WAN edge peer with the LAN on a dedicated subnet works well too. The latter saves one less subnet per occurance within the topology.

If you have dual LAN routers, also best, if possible, to have dual links from each WAN router. That way you don't lose half your WAN bandwidth with loss of a LAN router.

No, don't believe GLBP has an advantage over OSPF if you have a LAN router in front of the WAN routers and if you have equal cost paths. If you need to support unequal cost paths, believe then GLBP could help. Or, again, in situations where you don't have an internal LAN router to provide ECMP.

With regard to remote trouble shooting, you can also wire one router's aux port to the other's console port and use reverse telnet.

With multiple paths, you can also perform IOS reloads during business hours. (Best to cost the links such that the target router stop forwarding traffic before doing the reload.)

New Member

Re: Dual 3845 branch edge with OSPF

Thanks. I like the trick of crossing the aux ports to support remote telnet between devices as well.

Mark

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