Same Network on Multiple Ethernet Interfaces

Answered Question
Dec 3rd, 2008
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I have a 2800 with 2 Ethernet interfaces. I have a stack of 3750s (layer 2) connecting to the 2800. I would like to connect both 2800 Ethernet interfaces to the 3750 stack for redundancy. 1 connection can be active while the other is a standby or the connections can load share. How should I do this?

My first thought was HSRP. The router complains because the 2 NICs are on the same network.

I have considered IP unnumbered on the Ethernet interfaces and would create a loopback interface. I haven't tested this.

Additional information. I have 2 networks (voice and data). In my current set up, I use 1 Ethernet interface at the router which has been divided into sub-interfaces (1 data and 1 voice).

Correct Answer by Joseph W. Doherty about 8 years 7 months ago

A 2800 with the necessary IOS can Etherchannel its two Ethernet ports. You should be able to make a cross stack member connection on the 3750s.

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Overall Rating: 4.5 (2 ratings)
Richard Burts Wed, 12/03/2008 - 14:22
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Do not spend much time considering ip unnumbered. It will not do what you want it to do (might work if the interfaces were point to point serial but unnumbered on Ethernet is different).

If you really want both interfaces on the router to be active then what you should consider is configuring Integrated Routing and Bridging. With IRB you configure bridging on the Ethernet interfaces (which allows them to be active in the same subnet) and you configure a virtual interface (BVI) which links to the bridged interfaces and the BVI is where you configure IP (so the subnet appears on a single interface).

I have not tested IRB where you are using subinterfaces (to accomodate trunking from the switch) and do not know how well it works.

While IRB is probably the best alternative for having both interfaces active, I would suggest that the benefit of doing this may not be worth the increase in complexity of the router environment (and the impact of waiting for spanning tree convergence when the active interface fails).



glen.grant Wed, 12/03/2008 - 17:47
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Very cool you learn something new everyday here. Have to keep that in mind.

Joseph W. Doherty Wed, 12/03/2008 - 18:17
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Indeed, can't really take credit for this one. Learned it myself on these forums (along with many other interesting tidbits).


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