In order to truly understand your interfaces, you’ll need to make use of the show interface command. By specifying an interface along with the command, you’ll be provided with statistical information, the interface’s MAC address, logical addresses, encapsulation, and most importantly, whether the interface is functioning correctly or not.
The show interface command is issued from the privileged EXEC mode prompt. In order to view the configuration of our router’s Ethernet 0 interface, use the show interface e0 command.
Router#show interface e0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 00e0.f751.d6af (bia 00e0.f751.d6af)
Internet address is 192.168.1.46/24
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
Encapsulation ARPA, loopback not set, keepalive set (10 sec)
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 2000 bits/sec, 3 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
15248 packets input, 1718569 bytes, 0 no buffer
Received 5405 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 input packets with dribble condition detected
12122 packets output, 4045434 bytes, 0 underruns
0 output errors, 0 collisions, 4 interface resets
0 babbles, 0 late collision, 1 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
It’s important to be familiar with the information provided by the show commands. When looking at the output of the show interface e0 command, a variety of information is provided, including the interface IP address, MAC address, statistics, and so forth. However, the first line contains information that is absolutely critical – it tells us about both the physical and data link characteristics of the interface.
The first portion, Ethernet 0 is up, refers to the Physical layer characteristics of the interface. In this case, it means that the link is receiving a carrier detect signal – in other words, our Ethernet interface is plugged into a working switch or hub. If the message had been Ethernet 0 is down, that would generally indicate a physical connectivity problem, such as the cable being disconnected.
The second portion, line protocol is up, refers to the Data Link layer characteristics of the interface. Keepalives are the messages sent over a link to ensure that a given interface is usable. If keepalives are functioning correctly, the line protocol is considered to be up.
There are four possible entries that you may come across when using the show interface command. Each is outlined below. Although this example uses an Ethernet interface, it could just as well be the output for a serial interface.
- Ethernet0 is up, line protocol is up. Both the Physical and Data Link characteristics of the interface are functioning correctly.
- Ethernet0 is down, line protocol is down. This message usually represents a physical interface problem. For example, the cable may be disconnected. This problem can also occur if this interface is connected to another router whose interface has been shut down using the shutdown command.
- Ethernet0 is up, line protocol is down. In this case, Physical layer connectivity is obviously not the issue. The line protocol being down is usually related to either a clocking issue (such as with keepalives) or a mismatch between the frame types being used on connected devices. For example, one router being configured to use ARPA frames, and another to use SNAP.
- Ethernet0 is administratively down, line protocol is down. This output means that a local interface has been manually shut down using the shutdown command. In the example below, the shutdown command is issued for interface serial 0, followed by the show int s0 command.
Router(config)#int s0
Router(config-if)#shutdown
Router(config-if)#^Z
Router#show int s0
Serial0 is administratively down, line protocol is down
