Router - "team" two ethernet interfaces

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Feb 6th, 2009
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I'm trying to come up with a way to make our router have two paths to the same internal network. In case one switch fails, everything can still get to the router through the other switch. Everything behind the switches has a cat connection to both switches.


It's like this:


WAN

|

Router(2811 ISR)

|...............|

switch switch


Everything on the switches uses a 10.10.1.1 or 172.16.1.1 address on the router as it's default gateway. (one subnet is data, other's voice)


How can I setup the router so both those addresses can be assigned either at the interface, sub-interface, or something else, and both e/0 and e/1 can utilize them? Is this possible?

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Richard Burts Fri, 02/06/2009 - 13:01
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Scott


Ordinarily you can not configure 2 Ethernet interfaces on a router to the same subnet (you get error messages about overlapping addresses and the configuration attempt is rejected). There is a way to make it work but it is a bit complicated. It involves the configuration of IRB. In Integrated Routing and Bridging you would configure bridging, configure a bridge group, on the FastEthernet interfaces do not configure an IP address but assign the bridge group on each interface to activate bridging on the interfaces. Then you configure a Bridged Virtual Interface. On the BVI you configure the IP address and the parameters to route IP over the bridged interfaces.


This works with an access port. I assume from your description that the link from the switches to the router is a trunk. Is that correct? I have never tried it but assume that you might be able to do the bridging on the VLAN subinterfaces. And even if it does work I question whether the complexity that it introduces is worth it.


Perhaps another solution to consider would be to have the switches be layer 3 switches, have the data and the voice VLANs and subnets on the switch, let the switch route between the VLANs, and have the links from the switches to the router be routed links (in which case each link would be a separate subnet) and not have the problem of overlapping addresses. This would be a much more effective redundancy solution.


HTH


Rick

cdusio Fri, 02/06/2009 - 15:06
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You need to use a BVI. Then you can have both interfaces connected to the same switch and the routing point moves to the BVI interface.



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