Cisco Router — T1 Back-to-Back

For those studying for CCNA and CCNP certifications, a lab is an important part of the learning process. More and different types of interfaces are always a way to add complexity and challenge to a given topology. While standard serial connections are quite common in labs, back to back connections using T1 interfaces can provide a similar experience. Furthermore, WIC-1DSU and WIC-1DSU-V2 cards can often be quite inexpensive on the secondary market.

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

T1 Crossover CableAs seen in the image, a T1 crossover cable requires that pins 1 and 2 are connected to the opposite end on pins 4 and 5 respectively. Unlike the back-to-back serial cables (DTE to DCE), neither end of the T1 crossover cable has any special characteristics. However when the T1 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 t1 clock source internal
R1(config-if)service-module t1 timeslots 1-24 speed 64
R1(config-if)service-module t1 framing esf
R1(config-if)service-module t1 linecode b8zs
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 t1 clock source line
R2(config-if)service-module t1 timeslots 1-24 speed 64
R2(config-if)service-module t1 framing esf
R2(config-if)service-module t1 linecode b8zs
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 T1 crossover cable should bring up both interfaces. When looking at the output of show running-config, many of these options are configuration defaults and may not be shown.

Default Configuration of Serial Interfaces

  • service-module t1 clock source line
  • service-module t1 timeslots 1-24 speed 64
  • service-module t1 framing esf
  • service-module t1 linecode b8zs
  • no ip address
  • encapsulation hdlc
  • shutdown (shutdown is default but IS visible in running-config)

This configuration is a working sample. 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 T1 cards. Each model is dependent on certain hardware and/or software combinations. There are also models that are known as T1 “controllers” and require the configuration of a channel-group. A similar configuration will also allow 56K interfaces to communicate directly with one another.

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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