Serial Interface Configuration: Connecting Two Routers Back-to-Back
Lab Objective
- Configure two routers connected directly through a serial cable
🛠️ Equipment Needed for This Lab
- 2 Cisco routers with serial interfaces
- Back-to-back serial cable
- 1 PC with PuTTY installed
In this lab, we’ll connect and configure two routers — Router1 and Router2 — directly through their serial interfaces.
Router Configuration Overview
| Router Name | Interface Name | IP Address | Interface Type |
|---|---|---|---|
| Router1 | Serial 2/0 | 192.168.1.1 | DCE |
| Router2 | Serial 2/0 | 192.168.1.2 | DTE |
Step 1: Connect to the Router
Connect to the router through PuTTY and enter User Mode. What do you see at the command prompt?
If you see the prompt below, you’ve successfully entered User Mode. If not, check the console cable connection between the router and PC.
Router>
Step 2: Configure the Hostname on Router1
After connecting to Router1, run these commands:
Router#config t Router(config)#hostname Router1 Router1(config)#
The router hostname changes from Router to Router1.
Step 3: Configure the Serial 2/0 Interface on Router1
Now configure the Serial 2/0 interface on Router1:
Router1(config)#interface serial 2/0 Router1(config-if)#ip address 192.168.1.1 255.255.255.0 Router1(config-if)#clock rate 64000 Router1(config-if)#no shutdown Router1(config-if)#exit Router1(config)#exit Router1#
| Command | Description |
|---|---|
| Router1#config t | Moves from Privilege Mode to Global Configuration Mode. |
| Router1(config)#interface serial 2/0 | Enters Interface Configuration Mode for Serial 2/0. |
| Router1(config-if)#ip address 192.168.1.1 255.255.255.0 | Assigns IP 192.168.1.1 with mask 255.255.255.0 to Serial 2/0. |
| Router1(config-if)#clock rate 64000 | Sets the clock rate to 64000. Required because the DCE end of the cable connects to this interface. |
| Router1(config-if)#no shutdown | Enables the Serial 2/0 interface. Router interfaces are shut down by default. |
Step 4: Save the Configuration on Router1
To save the configuration permanently, copy the running-config from RAM to the startup-config in NVRAM:
Router1#copy running-config startup-config
Why this matters: Copying from running-config to startup-config preserves your configuration across reboots. If you leave the config only in RAM, it’s wiped when the router powers off.
Step 5: Configure the Hostname on Router2
After connecting to Router2, run these commands:
Router#config t Router(config)#hostname Router2 Router2(config)#
The router hostname changes from Router to Router2.
Step 6: Configure the Serial 2/0 Interface on Router2
Now configure the Serial 2/0 interface on Router2:
Router2(config)#interface serial 2/0 Router2(config-if)#ip address 192.168.1.2 255.255.255.0 Router2(config-if)#no shutdown Router2(config-if)#exit Router2(config)#exit Router2#
| Command | Description |
|---|---|
| Router2#config t | Moves from Privilege Mode to Global Configuration Mode. |
| Router2(config)#interface serial 2/0 | Enters Interface Configuration Mode for Serial 2/0. |
| Router2(config-if)#ip address 192.168.1.2 255.255.255.0 | Assigns IP 192.168.1.2 with mask 255.255.255.0 to Serial 2/0. |
| Router2(config-if)#no shutdown | Enables the Serial 2/0 interface. Router interfaces are shut down by default. |
Note: Because the DTE end of the cable connects to Serial 2/0 on Router2, no clock rate is needed on this interface.
Step 7: Save the Configuration on Router2
Save Router2’s configuration to NVRAM:
Router2#copy running-config startup-config
Step 8: Display Serial Interface Information with show interface serial
Run the following command to display details about Serial 2/0 on the router:
Router2#show interface serial 2/0
Study the output and answer these questions:
- What appears after
Serial 2/0 is? - What appears after
line protocol is? - What appears next to Internet Address?
- What appears next to Encapsulation?
Step 9: Test Connectivity Between Router1 and Router2 with PING
To test connectivity between Router1 and Router2, run the following command on Router1:
Router1# ping 192.168.1.2
What do you see in the output?
Now run the reverse test — from Router2 to Router1:
Router2# ping 192.168.1.1
What do you see in the output?
Success indicator: A successful ping shows five exclamation marks (!!!!!) with a 100% success rate. Failures show periods (.....).
DCE vs. DTE at a Glance
| Feature | DCE (Router1) | DTE (Router2) |
|---|---|---|
| Clocking | Provides clocking | Receives clocking |
| Command Needed | clock rate 64000 |
None |
| IP Address | 192.168.1.1 | 192.168.1.2 |
| Interface | Serial 2/0 | Serial 2/0 |
Full Command Reference
| Command | Description |
|---|---|
| Router#config t | Moves from Privilege Mode to Global Configuration Mode. |
| Router(config)#hostname Router1 | Changes the router hostname to Router1. |
| Router1(config)#interface serial 2/0 | Enters Interface Configuration Mode for Serial 2/0. |
| Router1(config-if)#ip address 192.168.1.1 255.255.255.0 | Assigns IP address and mask to the interface. |
| Router1(config-if)#clock rate 64000 | Sets the clock rate on the DCE interface. |
| Router1(config-if)#no shutdown | Enables the interface. |
| Router1#copy running-config startup-config | Saves the configuration to NVRAM. |
| Router2#show interface serial 2/0 | Displays Serial 2/0 interface details. |
| Router1#ping 192.168.1.2 | Tests connectivity to Router2. |
| Router2#ping 192.168.1.1 | Tests connectivity to Router1. |
✅ Key Takeaways
- The DCE end of a back-to-back serial cable requires the
clock ratecommand. - The DTE end does not require clock rate configuration.
- Router interfaces are shut down by default — enable them with
no shutdown. - Both routers must be on the same subnet to communicate (192.168.1.0/24).
copy running-config startup-configsaves the configuration to NVRAM.- Use
show interface serial 2/0to verify interface status and line protocol. - Use
pingto test connectivity between the two routers. - Five exclamation marks (
!!!!!) indicate a successful ping.
