Cisco Router — 56K Back-to-Back

Like connecting T1 cards directly, back-to-back 56K communication requires a special cable. Also like T1 crossover cables, anyone having bulk networking cable, some RJ45 connectors and a pair of crimpers can make this 56K crossover cable. The specific pinout is shown in the image below (imagine the tab facing away from you).

56K CrossoverAs seen in the image, a 56K crossover cable requires that pins 1 and 2 are connected to the opposite end on pins 7 and 8 respectively. Unlike the back-to-back serial cables (DTE to DCE), neither end of the 56K crossover cable has any special characteristics. However when the 56K interfaces are configured, one end does need to be configured to supply the clock to the other end.

T1 Serial

R1 Configuration

R1(config)interface serial 0/0
R1(config-if)service-module 56k clock rate 56
R1(config-if)service-module 56k clock source internal
R1(config-if)ip address 10.1.1.1 255.255.255.0
R1(config-if)encapsulation ppp
R1(config-if)no shut

R2 Configuration

R2(config)interface serial 0/0
R2(config-if)service-module 56k clock rate 56
R2(config-if)service-module 56k clock source line
R2(config-if)ip address 10.1.1.2 255.255.255.0
R2(config-if)encapsulation ppp
R2(config-if)no shut

The above configuration, combined with the previously described 56K crossover cable should bring up both interfaces. It is worth noting that the interfaces on either end of the link must be a 56K interface (a T1 card CANNOT be used as a surrogate on one end).

This configuration is a working sample. Like the example I gave with T1 back-to-back connections, it is possible to use these types of interfaces with HDLC and Frame-Relay labs as well. One words of caution is that Cisco has several different models of interface cards. Each model is dependent on certain hardware and/or software combinations that should be researched for compatibility.

About Paul Stewart, CCIE 26009 (Security)

Paul is a Network and Security Engineer, Trainer and Blogger who enjoys understanding how things really work. With over 15 years of experience in the technology industry, Paul has helped many organizations build, maintain and secure their networks and systems.
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One Response to Cisco Router — 56K Back-to-Back

  1. J,D. says:

    Thanks for the info worked like a charm. I did however test and confirm that you can set the clock rate to 64 and it works fine. It gives a warning when you do it “WARNING – 64k mode will not work in back-to-back DDS” but it is working.

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