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Chapter 4

Introduction to the Cisco IOS

RouterA#sh int e0

Ethernet0 is up, line protocol is up

The first parameter refers to the Physical layer, and it's up when it receives

carrier detect. The second parameter refers to the Data Link layer, and it
looks for keepalives from the connecting end. (Keepalives are used between
devices to make sure connectivity has not dropped.)

RouterA#sh int s0

Serial0 is up, line protocol is down

If you see the line is up but the protocol is down, as shown above, you are

experiencing a clocking (keepalive) or framing problem. Check the keepalives
on both ends to make sure that they match, the clock rate is set, if needed,
and the encapsulation type is the same on both ends. This would be consid-
ered a Data Link layer problem.

RouterA#sh int s0

Serial0 is down, line protocol is down

If you discover that both the line interface and the protocol are down,

it's a cable or interface problem. This would be considered a Physical layer
problem.

If one end is administratively shut down (as shown next), the remote end

would present as down and down.

RouterB#sh int s0

Serial0 is administratively down, line protocol is down

To enable the interface, use the command no shutdown from interface

configuration

mode.

The next show interface serial 0 command demonstrates the serial

line and the maximum transmission unit (MTU)--1500 bytes by default. It
also shows the default bandwidth (BW) on all Cisco serial links: 1.544Kbs.
This is used to determine the bandwidth of the line for routing protocols like
IGRP, EIGRP, and OSPF.

Another important configuration to notice is the keepalive, which is

10 seconds by default. Each router sends a keepalive message to its neighbor
every 10 seconds, and if both routers aren't configured for the same keepalive
time, it won't work.

You can clear the counters on the interface by typing the command clear

counters

Router#sh int s0

Serial0 is up, line protocol is up

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