Hands-On Lab: Configuring Back-to-Back Serial Connections

Lab Objective

Connect two routers using a back-to-back serial cable, correctly configure clocking on the DCE side, assign IP addresses, and verify the link comes up correctly.

Lab Purpose

Back-to-back serial connections are used constantly in lab and testing environments to simulate a WAN link without needing an actual service provider circuit. Understanding which end requires clock rate configuration — and what happens when it is missing — is a foundational serial troubleshooting skill.

Lab Topology

R1 ---- Serial0/0/0 (DTE) ------ Serial0/0/0 (DCE) ---- R2

R1: 10.5.5.1/30
R2: 10.5.5.2/30

The cable's DCE end is physically connected to R2

Task 1: Identify the DCE End

Use a show command on both routers to determine which router's interface is physically connected to the DCE end of the cable.

Task 2: Configure IP Addressing

Assign the addresses shown in the topology to each router's serial interface.

Task 3: Configure Clock Rate on the DCE Side Only

Apply an appropriate clock rate only to the DCE-side interface.

Task 4: Verify the Link Comes Up

Confirm both interfaces show as up/up, and that R1 can ping R2.

Task 5: Observe the Effect of a Missing Clock Rate

Remove the clock rate from the DCE interface and observe the resulting interface state before restoring it.

Solution and Verification

R1# show controllers serial0/0/0 | include cable

DTE V.35 TX and RX clocks detected.

R2# show controllers serial0/0/0 | include cable

DCE V.35 TX and RX clocks detected.

R1(config)# interface serial0/0/0
R1(config-if)# ip address 10.5.5.1 255.255.255.252
R1(config-if)# no shutdown

R2(config)# interface serial0/0/0
R2(config-if)# ip address 10.5.5.2 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown

R1# show ip interface brief

Serial0/0/0    10.5.5.1    YES manual up    up

R1# ping 10.5.5.2

!!!!!
Success rate is 100 percent (5/5)

R2(config)# interface serial0/0/0
R2(config-if)# no clock rate

R1# show interfaces serial0/0/0

Serial0/0/0 is up, line protocol is down
-- Notice: the interface itself is "up"
-- (the cable is physically connected), but
-- "line protocol is down" -- without clocking
-- from the DCE side, no signal timing exists
-- for either side to actually communicate

R2(config)# interface serial0/0/0
R2(config-if)# clock rate 64000
-- restoring clocking brings line protocol
-- back to "up" within a few seconds

Key Takeaway

Only the DCE end of a back-to-back serial connection requires a configured clock rate — configuring it on the DTE side has no effect, and a missing clock rate on the DCE side produces the distinctive "up, line protocol down" state rather than a fully down interface.