Cisco lab - fractional T1 interfaces

crtplague

Hi all, I know this is not directly certification related but I have a burning question about hooking up Cisco fractional T1 interfaces back to back:

In my home BSCI lab, I'm trying to create an OSPF stub area by hooking up two Cisco 2611 routers both using fractional T1 CSU/DSU interfaces with a T1 x-over cable. All the configurations are correct on both sides (clocking, clock rate, DCE/DTE, b8zs, esf, frame-relay encapsulation, line build-out set to "none", etc, etc). Ok, no problem there.


I played around with this for a while trying to get the two interfaces to come up but still showed "down & down" status. I Googled for a while and found the solution: "use a longer T1 cross-over cable". This fixed the problem and two interfaces came right up. My question is: why would a 20ft T1 cross-over cable make any difference compared with the 2ft that I was using before??? Both are known good working cables. I don't get it, why would the longer cable make everything work. Do the interfaces sense if there's a slightly higher attenuation? Cisco layer1 voodoo?


Any explanations would be great.

thanks!

crtplague

dtlokee

How do you know the original cable was good? I use 2-3 ft cables in my lab and don't have any issues. There is a setting to configure the cable length to short which may change things (lower output gain).

scheistermeister

It works like a thermos... Magic!

crtplague

The original cable works fine when I use it between two T1 interfaces on a pair of old Netopia routers. Guess I'll trying playing around with the line build-out.


Thanks!