Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

2 interfaces on the same subnet


I have a 1721 router at a client's site that has been doing NAT'ing and VPNs for the customers network. They have a 3com phone system, and 3com just donated a new firewall to the cause.

But now, everyone wants the VPNs and NAT moved to the firewall. So, the new firewall needs to be setup w/ a public and private IP address. In order to do this, I must change my router config.

My setup is this:

I have a T1 coming thru a patch panel, into the ethernet port on the router. From the router I have a cable from the fast ethernet port to the WAN port on the firewall.

The IPs I can use for the router are: x.x.x.82 (currently eth0) and x.x.x.83 (will be fast0). But since I can't have 2 subnets on the same router, I need to work around this.

I'm basically turning my nice router into just a bridge. However, I can't seem to get the commands right!

I need suggestions on what to do! Thanks for the input!!



Re: 2 interfaces on the same subnet

Hmm. I believe that if you're turning the router into a bridge, you don't need IP addresses on the interfaces. At least not both of them. Why not put the IP address on the FastE and do an "ip unnumbered" on the Eth interface? Then configure them in the same bridge-group.

I am curious though, why even have the router in there if its just going to be bridging traffic? Since its an Eth port on the T1 side, I'm assuming you have another router terminating the T1 since a T1 would take a serial interface. What else is this router doing or can you just take it out of there?