Cisco 3640, 4 T1 CSU/DSU Interfaces One Subnet?

[Podolak]

Hey All,

We have a Cisco 3640 Router with 4 T1 CSU/DSU Interfaces and an ethernet port. What we want is all three interfaces to be on the same subnet. Is this possible? Every time I try and configure this I get the subnet overlap error. Any ideas?

 

[Garion]

The most typical arrangement is for each interface to be on it's own subnet. Why do you want them all on the same subnet?

- Jason

 

[Podolak]

God if I knew the answer to that...

because they don't want to re-arrange their addressing scheme acutally...

As silly as it sounds that is pretty much what it comes down to. I think I'm just going to have to explain it to them that they really have no choice but to do just that.

 

[spidey07]

that HAVE to be separate subnets.

typically you would use a special Class C address set aside for WAN addresses, like

10.254.254.0/24

and then each interace on a WAN would be 10.254.254.1/30, 10.254.254.5/30, etc

If you don't want them in the same subnet (this is really crazy IMHO) then you would bridge all of your WAN connections. At that point all sorts of trouble and broadcast storms with only one T1 being actually used would be what happens.

 

[Garion]

Does each T1 go to a different site? They might be using something overly complex like IRB that could be causing the confusion..

- G

 

[Podolak]

Yes, they all go to different locations

 

[w0ss]

Do what the pros do and just make up IP's for the serial links. Works everytime.

 

[ToeJam13]

The problem is that the Cisco routers don't like to act as a Bridge. I have a 2501 in the "outdated technology bin", and it requires each serial link to be a serperate subnet for routing purposes. The only way it'll let me get around it is if I multiplex the interfaces and use MPPP.

This is fairly common for Cisco equipment. Of course, there are easy ways to get around it. You can use IP Encapsulation to extend the same subnet to all of your remote offices. By enabling compression, any overhead created by the encapsulation headers will be minimized.