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We recently started leasing a small new site and part of the agreement was to have the owner of the building provide a point to point link between our main site and this building and we would purchase configure and maintain the hardware. The job fell on me to configure the two routers (1841s with CSU/DSU cards).

I unfortunately am not very Cisco savvy and I'm hoping to find some help here. I'm hoping to configure the two routers to essentially work like a standard ethernet cable. They need to be on the same subnet as one of our other departments (not NAT'd). The only examples I can find online involve both ends of the router being on separate subnets.

On our main site I have a port ready with the subnet I need. Can anyone out there provide me with some help?

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up vote 2 down vote accepted

The terminology you're looking for is "bridging" when you say "essentially work like a standard ethernet cable". Beware that briding Ethernet over a slow WAN link may result in unacceptable utilization of the WAN link with broadcast traffic. That's why we typically route traffic across slow WAN links because, by default, routers don't forward broadcast traffic.

You want something like:

interface Serial 0
 bridge-group 1
 no ip address

interface FastEthernet 0
 bridge-group 1
 no ip address

interface BVI1
 ip address x.x.x.x y.y.y.y

ip route z.z.z.z

On those 1841's the "Serial 0" and "FastEthernet 0" interfaces might have slightly different names, but that's the gist of it.

Replaced "x.x.x.x" with an IP address on your subnet, "y.y.y.y" with the subnet mask for that subnet, and "z.z.z.z" with the default gateway for that subnet. You'll want to use a different "x.x.x.x" address for the remote end.

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Thanks for the answer, does the route belong on both of the routers? I configured them as shown and hooked them up with a T1 crossover. The links show as up but they are unable to ping each other. I haven't tried sending other traffic over the link. – TrueDuality Aug 9 '10 at 18:20
Never mind got it working, I ended up blowing away the configs that I had tried. Something in there was not making it happy. All is well now, I didn't use the routing commands though and it seems quite happy without them. – TrueDuality Aug 9 '10 at 19:29
@TrueDuality: Without the "ip route z.z.z.z" command the router won't be accessible to administer from IPs off-subnet. That may not matter to you, but might, too. – Evan Anderson Aug 9 '10 at 19:43
I thought that was handled through ip default-gateway for management traffic and the route command for the internal routing of packets when the router is acting as ... well a router. – TrueDuality Aug 10 '10 at 18:09

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