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.