HSRP on single router

Answered Question
Mar 2nd, 2007

RE: non-standard HSRP configuration

I have a single router connected with two separate cables to a redundant switch pair (A&B) uplinks...can I setup HSRP on the router to failover to either router port in the event of a cable or switch failure? Is this supported?

I have this problem too.
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Correct Answer by Richard Burts about 9 years 6 months ago

Dave

Configuration of HSRP between two interfaces on the same router is not supported and will not work. One of the restrictions in IOS is that you can not configure two active Ethernet interfaces in the same subnet. And the essence of HSRP is to have two active Ethernet interfaces in the same subnet talking to each other.

If you really want to connect two router interfaces to redundant switches and have the router interfaces back each other up, then you could try configuring bridging on both router interfaces. This would allow both interfaces to be active and Spanning Tree would provide the redundancy. To be able to route traffic from the interfaces you would need to configure IRB with a BVI interface to allow IP traffic from the router interfaces to be routed.

I would not recommend this approach, but it is the way to have both interfaces on the same router back each other up.

HTH

Rick

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Correct Answer
Richard Burts Fri, 03/02/2007 - 12:51

Dave

Configuration of HSRP between two interfaces on the same router is not supported and will not work. One of the restrictions in IOS is that you can not configure two active Ethernet interfaces in the same subnet. And the essence of HSRP is to have two active Ethernet interfaces in the same subnet talking to each other.

If you really want to connect two router interfaces to redundant switches and have the router interfaces back each other up, then you could try configuring bridging on both router interfaces. This would allow both interfaces to be active and Spanning Tree would provide the redundancy. To be able to route traffic from the interfaces you would need to configure IRB with a BVI interface to allow IP traffic from the router interfaces to be routed.

I would not recommend this approach, but it is the way to have both interfaces on the same router back each other up.

HTH

Rick

Richard Burts Mon, 03/05/2007 - 14:18

Dave

Thanks for using the rating system to indicate that a solution is found for your problem. (and thanks for the rating) It makes the forum much more useful when people can read about a problem and can know that they will read a solution to the problem.

I encourage you to continue your participation in the forum.

HTH

Rick

rts-net-support Tue, 03/06/2007 - 05:58

I just want to point out a problem that I had with the BVI configuration. I used to have the same situation, one router with 2 network interfaces connected to a pair of switches. I had the BVI interface up and everything seemed to work fine. However, once the traffic built up the performance of the router went down. Once I had about 15Mbps traffic through the BVI interface, the router's CPU was at 100%. This was a 2621XM. Once I removed the BVI, CPU dropeed to 20% or so for the same amount of traffic, or even higher.

So either the bridging stuff really impacts on performance or something else was happening.

Richard Burts Tue, 03/06/2007 - 07:47

I would assume that traffic through IRB is process switched. And certainly traffic that is process switched will impact performance of the router since process switching uses cycles in the router CPU.

This would be one of several reasons that I would not recommend the implementation being discussed. But if it is decided that they want to use 2 interfaces on the router to connect to switches and to back each other up, then this is the implementation that will make that work.

HTH

Rick

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