ethernet redundancy with interfaces on same layer 3 device

Unanswered Question
Dec 7th, 2007

I have a 3725 that will host a DS3 connection to a remote site. I want to connect the 2 onboard ethernet interfaces to seperate 6500's at my core, for connection redundancy. HSRP will not work because it will not allow me to put common subnet on different interfaces of same device. Is there a way to do something like convert the layer 3 interfaces of the 3725 to layer 2 interfaces, and create a virtual layer 3 interface for the routing/eigrp?

I have this problem too.
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Jon Marshall Fri, 12/07/2007 - 11:22


A simpler solution would be to have each interface in it's own subnet using a /30. ie.

3745 -> 6500_1

3745 -> 6500_2

If you use a routing protocol such like EIGRP/OSPF you will have 2 equal cost paths to each remote network.

In addition if a link to one of the router interfaces fails because the other link will be in the routing table you will switch over almost immediately, a packet lost at most.

Finally you are not extending STP up to your WAN router assuming you can make the ports on your 6500 routed ports. If not you could use dedicated SVI's.



gerheauserm Fri, 12/07/2007 - 11:26

understand your theory, and had contemplated that. Was hoping to tie both of the ethernet interfaces on the 3725 into my existing backbone vlan that the core 6500's currently participate in. I understand the implications of extending STP into this router, and the necessary evils I am accepting if I go that route. If there is no other clean way to accomplish, than so be it. Just didn't want to create more vlan's and small subnets if I didn't need to.

Jon Marshall Fri, 12/07/2007 - 11:34

Okay, i guess you could try bridging the 2 interfaces together and creating a BVI, but i have no idea if you could run HSRP on this.

Perhaps someone else could help out with that.

I can't think of a "clean" way to achieve this though without going down the L3 route.

If your 6500 is IOS based you don't need to create any more vlans you can just do a "no switchport" on the relevant 6500 port. But you are right in that you would need more vlans if you are running hybrid.


Edison Ortiz Fri, 12/07/2007 - 12:09

Yes, you can run HSRP on the BVI interface but what are the two Layer3 links forming the redundancy on this process ?

gerheauserm Fri, 12/07/2007 - 12:22

Actually, if I could get the ethernet interfaces on the 3725 router into a common bridges environment, I would not need hsrp. could I not, bridge the 2 ethernet interface, essentially making them a layer 2 interface, connect each of them to my 2 core 6513's (they are running in IOS mode, so I would make the associate switchports on the 6513's access mode and have them participate in my backbone vlan), and use the virtual BVI interface to act as the eigrp neighbor interface on my 3725?

Edison Ortiz Fri, 12/07/2007 - 12:39

You will have to play with your STP topology. One of those layer2 ports in the 3725 may be in blocking mode due to STP.

Jon Marshall Fri, 12/07/2007 - 12:58


I guess you could although as Edison says you may need to manipulate the STP costs.

If that's the way you want to go then good luck but i am still struggling to see how this is a better, cleaner solution than the L3 way. i'm not saying this because you didn't accept the solution, that's fine, it's just that if i came across the setup you are proposing the first thought i would have is why did the network admin do that ?.

You don't need any more vlans to do the L3 solution, just 2 more subnets. But as i say good luck with whichever you choose to implement.



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