Saturday, December 14, 2013

Cisco: What Is A "Late Collision" In A Router Or Switch

When I posted the 'collision/duplex mismatch' posting (click here to see it), I asked some IT guys that I work with what the difference between a 'collision' and a 'late collision' is.  This caused quite a stir here, and in reality, none of us really knew the answer to that. So we did the research.  If you are wondering what the difference between a 'collision' and a 'late collision' is, here is the answer, per Cisco:

Late Collisions

To allow collision detection to work properly, the period in which collisions are detected is restricted (512 bit-times). For Ethernet, this is 51.2us (microseconds), and for Fast Ethernet, 5.12us. For Ethernet stations, collisions can be detected up to 51.2 microseconds after transmission begins, or in other words up to the 512th bit of the frame.
When a collision is detected by a station after it has sent the 512th bit of its frame, it is counted as a late collision.
Late collisions are reported by these error messages:
%AMDP2_FE-5-LATECOLL: AMDP2/FE 0/0/[dec], Late collision 
%DEC21140-5-LATECOLL: [chars] transmit error 
%ILACC-5-LATECOLL: Unit [DEC], late collision error 
%LANCE-5-LATECOLL: Unit [DEC], late collision error 
%PQUICC-5-LATECOLL: Unit [DEC], late collision error 
%PQUICC_ETHER-5-LATECOLL: Unit [DEC], late collision error 
%PQUICC_FE-5-LATECOLL: PQUICC/FE([DEC]/[DEC]), Late collision    
%QUICC_ETHER-5-LATECOLL: Unit [DEC], late collision error
The exact error message depends on the platform. You can check the number of excessive collisions in the output of a show interface ethernet [interface number] command.
router#show interface ethernet 0
Ethernet0 is up, line protocol is up 
  Hardware is Lance, address is 0010.7b36.1be8 (bia 0010.7b36.1be8)
  Internet address is 10.200.40.74/22
  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, 
     reliability 255/255, txload 1/255, rxload 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 00:00:06, output hang never
  Last clearing of "show interface" counters never
  Input queue: 1/75/1/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: random early detection(RED)
  Output queue :0/40 (size/max)
  5 minute input rate 1000 bits/sec, 2 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     2058015 packets input, 233768993 bytes, 1 no buffer
     Received 1880947 broadcasts, 0 runts, 0 giants, 1 throttles
     3 input errors, 0 CRC, 0 frame, 0 overrun, 3 ignored
     0 input packets with dribble condition detected
     298036 packets output, 32280269 bytes, 0 underruns
     0 output errors, 10 collisions, 0 interface resets
     0 babbles, 0 late collision, 143 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out
Note: The station that reports the late collision merely indicates the problem; it is generally not the cause of the problem. Possible causes are usually incorrect cabling or a non-compliant number of hubs in the network. Bad network interface cards (NICs) can also cause late collisions.

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