Cisco Show Interface Command on Routers and Switches Explained

One of the most useful and popular commands used on Cisco devices is the “show interface” command.

It can be very useful at troubleshooting connectivity issues and physical port issues, check the status of physical ports, watch how much traffic is passing through the interface, which IP address is assigned to the interface (for Layer3 interfaces) etc.

This command works on both Cisco Switches and Cisco Routers and has the same functionality on both types of devices.

In this article we will discuss and explain the “show interface” command on both routers and switches, so let’s get started.

Topology Diagram

Here is the topology used to gather the various output data for each device in this article:

Show Interface on Cisco Switches

On a Layer 2 switch we can check the status and various other counters and metrics for each physical ethernet interface or for every interface on the device.

I usually start first with the following command:

Switch0# show interfaces status

Port    Name               Status       Vlan       Duplex  Speed Type

Fa0/1                        connected    1          auto    auto  10/100BaseTX
Fa0/2                        connected    1          auto    auto  10/100BaseTX
Fa0/3                        connected    1          auto    auto  10/100BaseTX
Fa0/4                        notconnect   1          auto    auto  10/100BaseTX
Fa0/5                        notconnect   1          auto    auto  10/100BaseTX
Fa0/6                        notconnect   1          auto    auto  10/100BaseTX
Fa0/7                        notconnect   1          auto    auto  10/100BaseTX
Fa0/8                       notconnect   1          auto    auto  10/100BaseTX

The above gives me a quick status update of each available interface on the switch, on which VLAN it belongs, speed settings etc.

Now if you want to dive deeper in the counters for each physical interface, we can be more specific as shown below:

Switch0# show interfaces FastEthernet 0/1

FastEthernet0/1 is up, line protocol is up (connected)
  Hardware is Lance, address is 000b.be66.0b01 (bia 000b.be66.0b01)
 BW 100000 Kbit, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:08, output 00:00:05, output hang never
  Last clearing of “show interface” counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue :0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     956 packets input, 193351 bytes, 0 no buffer
     Received 956 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     2357 packets output, 263570 bytes, 0 underruns
     0 output errors, 0 collisions, 10 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

The “Show Interface” command above which includes the specific interface number (FastEthernet 0/1), shows much more information. The most important data you need to take into consideration are the following:

FastEthernet0/1 is up, line protocol is up (connected): This shows that the specific interface is connected (both physical access and line protocol are up), so it is ready to pass traffic.

Hardware is Lance, address is 000b.be66.0b01 (bia 000b.be66.0b01): Here you can see the MAC (hardware) address of the interface.

     reliability 255/255: If you have reliability 255/255 it means that there are no input and output errors on the interface. If you have interface errors the reliability factor will decrease.

  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     956 packets input, 193351 bytes, 0 no buffer

The above counters are useful to check how much traffic has passed through the interface the last 5 minutes.

2357 packets output, 263570 bytes, 0 underruns
     0 output errors, 0 collisions, 10 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

The above counters are also useful to watch how much traffic has passed through the interface, if there were any errors or frame collisions, buffer failures etc.

NOTE: The number of errors shown on each interface is a cumulative number which shows statistics (input/output errors etc) from the time the device was booted up. If you see a small number of errors among millions of packets, then you don’t need to worry. However, if the total number of errors is a big percentage compared to the number of packets, then you should investigate for possible interface problems.

Let’s now see another useful show interface command on switches:

Switch0# show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/3       on           802.1q         trunking      1
Port        Vlans allowed on trunk
Fa0/3       1-1005
Port        Vlans allowed and active in management domain
Fa0/3       1
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/3       none

If you want to see which interfaces are configured as “Trunk” ports, the above command will show you just this. As shown above, port Fa0/3 is configured as 802.1q Trunk which means it is connected to another switch in order to pass VLANs from one switch to another.

As you can see, VLANs 1-1005 are allowed to pass through the trunk connection.

Show Interface on Cisco Routers

On a Layer 3 router, the show interface command is equally important to obtain crucial information about each interface for various purposes.

Let’s see three cases of the command:

Router0# show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     192.168.1.254   YES manual up                    up
GigabitEthernet0/1     192.168.2.254   YES manual up                    up
GigabitEthernet0/2     unassigned      YES unset  administratively down down

I always start by getting a brief high-level picture of all interfaces with the “show ip interface brief” command.

As shown above, this shows me the IP address assigned to each physical interface, if the port is UP, if the protocol is UP etc.

For example, one interface might be connected to a Frame Relay connection. The physical port might be UP but the protocol might show Down.

Now, if you want to get more IP details and other statistics for a specific interface, run the following:

Router0# show ip interface GigabitEthernet 0/0

GigabitEthernet0/0 is up, line protocol is up (connected)
  Internet address is 192.168.1.254/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is disabled
  IP fast switching on the same interface is disabled
  IP Flow switching is disabled
  IP Fast switching turbo vector
  IP multicast fast switching is disabled
  IP multicast distributed fast switching is disabled
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  WCCP Redirect outbound is disabled
  WCCP Redirect inbound is disabled
  WCCP Redirect exclude is disabled

The command above shows statistics up to Layer 3 of the OSI model (Physical, Data Link and Network Layers). It shows the IP address configured and also if there are any Access Control Lists (ACL) applied to the specific interface with the below fields:

  Outgoing access list is not set
  Inbound  access list is not set

An ACL is a packet filtering mechanism which controls IP packets passing through the interface (block or allow) based on IP address and port numbers.

Now let’s see the generic show command which we described also on the switch section above: 

Router0# show interfaces GigabitEthernet 0/0

GigabitEthernet0/0 is up, line protocol is up (connected)
  Hardware is CN Gigabit Ethernet, address is 00e0.f9d3.3d01 (bia 00e0.f9d3.3d01)
  Internet address is 192.168.1.254/24
  MTU 1500 bytes, BW 1000000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is RJ45
  output flow-control is unsupported, input flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00,
  Last input 00:00:08, output 00:00:05, output hang never
  Last clearing of “show interface” counters never
  Input queue: 0/75/0 (size/max/drops); Total output drops: 0
  Queueing strategy: fifo
  Output queue :0/40 (size/max)
  5 minute input rate 1017 bits/sec, 0 packets/sec
  5 minute output rate 1017 bits/sec, 0 packets/sec
     886 packets input, 113108 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 watchdog, 1017 multicast, 0 pause input
     0 input packets with dribble condition detected
     886 packets output, 113108 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Just like the command executed on a switch, the same command on a Cisco router displays similar statistics such as:

• Status of interface (up or down).
• Port reliability which depends on the number of input/output errors.
• 5 minute input and output traffic rate.
• Number of packets received .
• Errors on the interface
• And much more.