The counters in show interfaces
show interfaces prints a lot, but the counters at the bottom tell you how healthy the link is. The ones to know:
| Counter | What it counts | Points to |
|---|---|---|
| input errors | All received frames that were bad (the total of the ones below and more) | A physical problem somewhere |
| CRC | Frames whose checksum didn't match: the bits got damaged on the way | Bad or too-long cable, electrical noise, duplex mismatch |
| runts | Frames shorter than 64 bytes | Collisions cutting frames short (duplex mismatch) |
| giants | Frames bigger than the maximum (usually 1518 bytes) | MTU settings that don't match |
| collisions | Frames that collided (normal in small numbers on half duplex) | Half duplex |
| late collisions | Collisions after the first 64 bytes: should never happen | Duplex mismatch, or a cable far too long |
| output errors | Frames the interface failed to send | Collisions or a failing port |
| interface resets | How many times the interface was restarted | Flapping link, or someone shutting it down |
A cable that is too long
PC1's cable below is a 150 m copper run, well past the 100 m limit. The link is up, but the switch port collects CRC errors and pings drop:
In NetLab
▶ R1 R1>enable R1#ping 192.168.1.10 repeat 20 Type escape sequence to abort. Sending 20, 100-byte ICMP Echos to 192.168.1.10, timeout is 2 seconds: ...!..!!.!..!!..!!.! Success rate is 45 percent (9/20), round-trip min/avg/max = 1/1/1 ms ▶ SW1 SW1>enable SW1#show interfaces fa0/1 | include errors|CRC 3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored 0 output errors, 0 collisions, 0 interface resets
Counters only ever go up
Counters add up from the moment the router started, so old errors can fool you. After fixing a problem, clear the counters, send some traffic, and check that they stay at zero. clear counters asks you to confirm with Enter:
In NetLab
▶ SW1 SW1>enable SW1#clear counters Clear "show interface" counters on all interfaces [confirm] *Mar 1 00:00:03.000: %CLEAR-5-COUNTERS: Clear counter on all interfaces by console SW1#show interfaces fa0/1 | include errors|CRC 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 output errors, 0 collisions, 0 interface resets
Tip: Click a cable in NetLab to see its type and length, and to replace it, just as you would with a cable tester and a spare cable in a real comms room.