120 Labs for Cisco CCNA 200-125 and CCENT Exams

Lab 91: Configuring Back-to-Back Serial Connections

Lab Objective:

The objective of this lab exercise is to configure back-to-back Serial interfaces between two Cisco routers. By default, router Serial interfaces receive their clocking information from an external device such as a CSU/DSU.

Lab Purpose:

Back-to-back Serial interface configuration is a fundamental skill. Because routers typically receive clocking from an external device such as a CSU/DSU, it is imperative to understand how to bring up a back-to-back Serial connection between two routers to set up your home lab, for example. As a Cisco engineer, as well as in the Cisco CCNA exam, you will be expected to know how to configure back-to-back Serial connections.

Certification Level:

This lab is suitable for both CCENT and CCNA certification exam preparation.

Lab Difficulty:

This lab has a difficulty rating of 3/10.

Readiness Assessment:

When you are ready for your certification exam, you should complete this lab in no more than 10 minutes.

Lab Topology:

Please use the following topology to complete this lab exercise:

Task 1:

Configure hostnames on R1 and R2 as illustrated in the topology.

Task 2:

Enable Serial interfaces on R1 and R2. The Serial0/0 interface on R2 is identified as the DCE in the topology. Use the appropriate show command to verify that this interface is indeed the DCE.

Task 3:

Configure the DCE interface on R2 to provide clocking to R1. The clock speed should be 256 Kbps. Remember that 1 Kbps = 1000 bps. Verify that R1 receives clocking information from R2.

Task 4:

Configure IP addressing on R1 and R2 Serial0/0 interfaces as illustrated in the topology.

Task 5:

Verify your interface status and ping between R1 and R2 to validate connectivity.

Configuration and Verification

Task 1:

For reference information on configuring hostnames, please refer to earlier labs.

Task 2:

R1(config)#interface serial0/0 
R1(config-if)#no shut 
*Mar  1 00:36:47.282: %LINK-3-UPDOWN: Interface Serial0/0, changed state to down 
R1(config-if)#end 
R1# 

R2(config)#interface serial0/0 
R2(config-if)#no shut 
*Mar  1 00:36:47.282: %LINK-3-UPDOWN: Interface Serial0/0, changed state to down 
R2(config-if)#end 
R2#show controllers serial 0/0 
Interface Serial0/0 
Hardware is PowerQUICC MPC860 
DCE V.35, no clock

NOTE: The show controllers  command will tell you whether the interface is the DCE side (which provides the clocking) or the DTE side (which receives the clocking) on a particular router interface. Note that GNS3 doesn’t use actual cables so there is no need to configure clocking.

Task 3:

R2#conf t 
Enter configuration commands, one per line.  End with CTRL/Z. 
R2(config)#interface serial0/0
R2(config-if)#clock rate 256000 
R2(config-if)#end 
R2#show controllers serial0/0 
Interface Serial0/0 
Hardware is PowerQUICC MPC860 
DCE V.35, clock rate 256000 

R1#show controllers serial0/2 
Interface Serial0/0 
Hardware is PowerQUICC MPC860 
DTE V.35 TX and RX clocks detected.

Task 4:

For reference information on configuring IP addressing, please refer to earlier labs.

Task 5:

R1#show ip interface brief 
Interface     IP-Address     OK? Method Status                Protocol 
Serial0/0     172.30.100.1   YES manual up                    up 

R1#ping 172.30.100.2 
Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 172.30.100.2, timeout is 2 seconds: 
!!!!! 
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms 

R2#show  ip interface brief 
Interface   IP-Address     OK? Method Status                Protocol 
Serial0/0  172.30.100.2    YES manual up                    up 

R2#ping 172.30.100.1

Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 172.30.100.1, timeout is 2 seconds: 
!!!!! 
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms

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