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Chapter 3 - Configuring Cisco Routers

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Lab Exercises
We will now alter the configuration to make things stop working and look at how these problems can be identified and resolved.
Make these changes to the lab environment and we will troubleshoot the network and get it back into a working state.
  1. Change the encapsulation on router 3 Serial 0 port to ppp.
  2. Change the polarity of the DTE/DCE cable, so that the end of the cable connected to the router 2 Serial 0 port is connected to the router 3 Serial 0 port.
  3. Configure the Ethernet 0 port on router 1 to be shut down.
  4. Change the Autonomous System number of the IGRP process on router 2 to 15.
  5. Disconnect the Ethernet 0 port on router 2 from the hub.
To effect the router configuration changes, perform the following:
Router3#conf t
Router3(config)#int s0
Router3(config-if)encapsulation ppp
Router1#conf t
Router1(config)#int e0
Router1(config-if)shutdown
Router2#conf t
Router2(config)#no router igrp 9
Router2(config)#router igrp 15
Router2(config-router)network 160.4.0.0
A troubleshooting procedure should follow the previously described OSI seven-layer model of communications. You need to check the Physical layer first, then the Data Link and so forth, until the system is communicating properly.
Overview of Physical Layer Troubleshooting
A visual check shows whether cables are connected, but may not show a cable break. The best way to determine whether a router port is physically connected is to use the show interface command and interpret the screen output. Perform the following on router 2:
Router2#show interface serial 0
Ethernet0 is up, line protocol down
This is the first line of the output, and shows you that the port is okay, but was not able to establish a protocol session. This is either due to an unplugged/broken cable, or a broken transceiver. In this case, the cable is simply unplugged; replugging the cable brings the line protocol up. The problem generated by change number 5 has been detected and corrected.
Next on the Physical level you should check the serial port connections. Entering the following in router 2 will bring up a screen display, the last line of which is as shown.
Router2#show interface serial 0
  .
  .
DCD=up DSR=up DTR=up RTS=up CTS=up
This indicates that the physical connectivity to this port is working, and all the expected EIA signals are present. The port is, however, showing that the line protocol is down. This is due to either clocking problems or mismatched encapsulation. To resolve this, we issue the show controllers serial 0 command on both router 2 and 3, and check that the router configuring its Serial 0 port as a DCE has the clockrate 64000 entry in its configuration. By changing the polarity of the DTE/DCE cable, we match the DCE port with the "clockrate 64000"-configured serial port and resolve the problem caused by change 2.
The final Physical layer issue to determine whether any ports that need to be in use are physically shut down. Enter the following on router 1.
Router1#show interface ethernet 0
Ethernet0 is administratively down, line protocol down
The first line of the screen output as shown indicates that the port has been shut down by the administrator, by the shutdown entry in the port's configuration. This can be fixed by issuing the no shutdown command in interface configuration mode, which resolves the problem caused by change 3.
Overview of Data Link Layer Troubleshooting
For Data Link layer troubleshooting, we refer to the router configuration. The only thing to check here is that connected ports share the same Data
Link layer encapsulation. By issuing the wri t command to both router 2 and 3, we see mismatched layer 2 encapsulations. By viewing the configurations, we see PPP as the encapsulation for Serial 0 on router3, but the Serial 0 port of router 2 shows no encapsulation. With no encapsulation, the Cisco HDLC default is used. Taking out the encapsulation ppp entry from the Serial 0 port of router 3 resolves problem 1.
Overview of Network Layer Troubleshooting
Once the layer 1 and 2 problems have been resolved, only the Network layer remains to be checked in our scenario. We know that with IGRP, routing information is exchanged only between systems belonging to the same Autonomous System number. By reviewing the router configurations, we see that router 2 has a different AS number than router 1 and 3. Changing the AS to match enables all routers to share route information, which resolves problem 4. All routers can now ping one other again, because IGRP can now update each router's routing table with the necessary information.

 


 
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