Learning All About Router

Examining the Interfaces

Routers can have many types of interfaces, such as token ring, FDDI, ethernet, serial, ISDN etc. We often want to view the status and settings. There are a few important commands we must know.

show interfaces is on of the more important commands.
Router#show interfaces
Ethernet0 is administratively down, line protocol is down
Hardware is Lance, address is 0060.5cc4.f445 (bia 0060.5cc4.f445)
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255Encapsulation ARPA, loopback not set, keepalive set (10 sec)
[ OUTPUT OMMITTED]
This command will produce output about each interface. In this case we see that Ethernet 0 is administratively down. That means that it is turned off with the shutdown command. The different status that can occur:

Ethernet 0 is Line protocol is Meaning administratively down down The interface is turned off with the shutdown command up down Cable is connected but keep alives are not being received. down down Cabling problem or no clock rate set on DCE. Or other router interface is shutdown. up up connected and receiving keep alives. This is what we want!!!


You can view particular intefaces with the command: show interface serial 0. Or any other interface. A handy command is show ip interface brief.
Router#show ip int brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 unassigned YES not set administratively down down
PCbus0 unassigned YES not set administratively down down
Serial0 unassigned YES not set up down
Router#


This allows you to rapidly see the status of all the interfaces.
Examining the Controllers
Controllers are the part of the interface that makes the physical connection. The most important to us is to find our what kind of cable is attached to a serial interface.
A DTE (data terminating equipment) cable is the normal cable you should use. Being DTE means you expect the other end to providing clocking.
A DCE data circuit-terminating equipment) means that this deving must provide the clocking on the wire.


The show controllers command will allow you to see if you are DCE or DTE.
Router#show controllers serial 0
HD unit 0, idb = 0xA2B58, driver structure at 0xA7020buffer size 1524 HD unit 0, V.35 DCE cablecpb = 0x42, eda = 0x2140, cda = 0x2000
Configuring the Interfaces
If an interface is administratively down. You must enter configuration mode, the enter interface configuration mode, and then issue the command no shutdown.
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface ethernet 0
Router(config-if)#no shutdown
Router(config-if)#%LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up%LINK-3-UPDOWN: Interface Ethernet0, changed state to up
Router(config-if)#endRouter#
If your interface is the DCE, you must provide clocking using the clock rate command.
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface serial 0
Router(config-if)#clock rate 56000Router(config-if)#end
Router#


It is often useful to put a description of what the interface is used for using the description command.
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int e0
Router(config-if)#description My Connection to the Engineering Hub
Router(config-if)#end
Router#
You can view your changes using show running-config or show interfaces or show controllers

Copyright (c) 2001 Boson Software, Inc. All Rights Reserved.