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Serial Port FAQ

Cisco Serial Port FAQ, tips and hints by wybnormal
Posted: 29 Jan 01

Cisco Serial notes FAQ

Can I connect my routers back to back?

Yes you can. There are a couple of ways to do this. With two routers you can have them back-to-back with or without a T1 link. One way is purchase the special cable that allows you to connect the routers together. A second way is to get a serial to V.35 DTE and DCE cable set and connects the two V.35 connectors together. A third way is to make a T1 back-to-back cable and then connect the two routers through the DSU/CSUs.

What are the Cisco cables for DTE to DCE back to back?

CAB-V35MT and CAB-V35FC (Cisco 2500 or other high density serial connectors)

If you are using a T1-WIC card, don't forget the following command:

interface s0
 service-module t1 timeslots 1-24

What pins do I need to connect for a T1 loopback cable?

Using a RJ45 and looking from the connector side up, with the cable end point towards yourself, count from left to right.
Pin 1 to 5
Pin 2 to 4
Pin 4 to 2
Pin 5 to 1

A useful troubleshooting aid is to make a single loopback connector, pins 1 -5 and 2-4  This will let you test the serial interface to the T1 interface termination.

This will let you bring up the T1 interface and test just as if you had a Telco pull point-to-point circuit.

Can I make a 56K back-to-back cable?

Sure, construct the 56K crossover (back to back) cable by using pins 1-7, 2-8, 7-1, 8-1

Any other things I need to know about a 56K back to back?

You cannot use a clock rate of 64, cannot use switched-56 or use the clock rate of auto on either side

Can I emulate a frame relay connection without using the phone company?

Absolutely.. you need a third Cisco router with TWO serial ports. You also need two back-to-back cables or 2 DTE and 2 DCE cables. The third router is configured using the command:

! Cisco dual serial port router
frame-relay switching ; enable frame switch
interface serial 0/0
 no ip address
 encapsulation frame-relay ; sets encapsulation to frame relay
clockrate 38400 ; gens clock rate of 38400 to DTE
 frame-relay lmi-type ansi ; sets ANSI type
 frame-relay intf-type dce ; sets DCE or DTE
 frame-relay route 100 interface serial 0/1 101 ; defines the PVC between ports S0/0 and S0/1
interface serial 0/1
 no ip address
 encapsulation frame-relay
 clockrate 38400
 frame-relay lmi-type ansi
 frame-relay intf-type dce
 frame-relay route 101 interface serial 0/0 100

To connect two HSSI ports back to back, use a null modem cable, part number [colorred]92-0727-01 and use the command: hssi internal-clock to turn on the internal clocking. Don't forget to turn it off when you remove the back to back cable.

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