Basic Checks on Cisco Switch Interfaces

Sometimes we need to know if there are some connectivity problems between the connected host and the switch interface.

The show logging command for Cisco IOS can be used to check if an interface was unavailable for a certain duration

Switch#show logging
Oct 10 01:53:43: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/1, changed state to down
Oct 10 01:56:14: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/1, changed state to up

In addition you will need to be familiar with the following show interfaces command. The following information is provided when you enter the command:

• Interface type
• Status
• Speed and duplex
• Encapsulation
• Errors on the interface
• The last time the interface was bounced
• The last time the error counters were reset
• Port utilization
• IP address, subnet mask, and MAC address
  

Switch#show interface GigabitEthernet 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 001c.5723.e901
Description: Connection to BP_CS-FW-1
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output 00:00:00, 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 187000 bits/sec, 81 packets/sec
5 minute output rate 207000 bits/sec, 124 packets/sec
3290283950 packets input, 2160027042 bytes, 0 no buffer
Received 10 broadcasts (0 multicast)
24 runts, 0 giants, 0 throttles
24 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
4053517837 packets output, 3285226057 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out

The main uses for the show interface command are to:

• Determine if the interface is up and if the protocol is up
• Ascertain if the interface has errors on it, especially CRC errors
• Find out the speed and duplex of the interface (for Ethernet interfaces)
  
• Learn the current utilization and utilization over the last 5 minutes
• Determine the last time an interface was bounced
  

To find out the transmit/receive rate of an interface, the packets in the hold queue, or the packets dropped from the queue, use the show interface summary command.

Switch#show interface summary

 *: interface is up
 IHQ: pkts in input hold queue     IQD: pkts dropped from input queue
 OHQ: pkts in output hold queue    OQD: pkts dropped from output queue
 RXBS: rx rate (bits/sec)          RXPS: rx rate (pkts/sec)
 TXBS: tx rate (bits/sec)          TXPS: tx rate (pkts/sec)
 TRTL: throttle count

  Interface              IHQ   IQD  OHQ   OQD  RXBS RXPS  TXBS TXPS TRTL
------------------------------------------------------------------------
  Vlan1                    0     0    0     0     0    0     0    0    0
* Vlan27                   0     0    0     0     0    1     0    0    0
* Vlan45                   0     0    0     0     0    0     0    0    0
* GigabitEthernet1/0/1     0     0    0     0 23000   13 44000   18    0
* GigabitEthernet1/0/2     0     0    0     0 74000   14 32000   21    0

For a quick idea on whether there are any CRC errrors on any of the interfaces, filter the show interfaces command using the pipe [|] and include option.

Switch#show interfaces | inc CRC
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored