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New Member

One Arm Routing!

Does anybody know how to configure one arm routing using sub-interfaces on a single ethernet interface on a Cisco 2500 router connected to a Catalyst 1924 or Catalyst 3524 switch? Also does anybody know if either of these switches support dual Vlan membership and Vlan 802.1q tagging for any given port?

I am trying to get a single ethernet interface on a 2500 to route between two seperate VLAN's on a 1900 or 3524 switch!

Thanks in Advance to anybody who can help!


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New Member

Re: One Arm Routing!

A 2500 router shouldnt be capable of a 100baseT interface since the ethernet port is fixed 10baseT. This is needed for the subinterfaces and encapsultion for vlan tagging.


Paul Aquino


Cisco Systems

New Member

Re: One Arm Routing!

I don't think you will be able to do that on a 2500. The router will need a FastEthernet port (not 10MB, but 100MB) port to support trunking. Also not all FastEthernet ports support trunking, for example on the older 1700 routers (1750) it had a fastethernet port but it did not support trunking.

Also the 3524 does support trunking.

-Ali Tehrani

New Member

Re: One Arm Routing!

Adrian, only fast ethernet ports support trunking. As far as I know, none of the 2500 series routers have the hardware or software to support trunking. You need a 2651 at least. If a 2500 is all you have, this what you can do:

1: Combine the two vlans into one.

2: Go ahead and program your devices on the two different IP subnets.

3: Program the router's ethernet interface with a secondary IP address.

4: make sure you issue the ip route-cache same-interface command.

Steps 1-3 sets up the basic connectivity. Step 4 gives you the performance. However, you still have one big broadcast domain. so if the goal is to decrease broadcast, then this trick will not help. If the goal is to help IP address migration, then this trick should work.

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