This document provides an overview of the different counters related to Ethernet collisions and how to troubleshoot problems with Ethernet collisions reported by the following error messages (depending on the platform):
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Ethernet uses CSMA/CD (Carrier Sense Multiple Access/Collision Detect) as its collision detection method. Here is a simplified example of Ethernet operation:
In sum, collisions are a way to distribute the traffic load over time by arbitrating access to the shared medium. Collisions are not bad; they are essential to correct Ethernet operation.
Some useful facts:
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) 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
The deferred counter counts the number of times the interface has tried to send a frame, but found the carrier busy at the first attempt (Carrier Sense). This does not constitute a problem, and is part of normal Ethernet operation.
As explained above, collisions do not constitute a problem. The collisions counter counts the number of frames for which one or more collisions occurred when trying to send them.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) 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
The collisions counter can be broken down into single collisions and multiple collisions, as in this output from the show controller command:
This means that eight (out of 10) frames have been successfully transmitted after one collision; the two other frames required multiple collisions to arbitrate access to the medium.8 single collisions, 2 multiple collisions
An increasing collision rate (number of packets output divided by the number of collisions) doesn't indicate a problem: it is merely an indication of a higher offered load to the network. An example of this could be because another station was added to the network.
There is no set limit for "how many collisions are bad" or a maximum collision rate.
As a conclusion, the collisions counter doesn't provide a very useful statistic when it comes to analyzing network performance or problems.
When a collision is detected by a station after it has sent the 512th bit of its frame, this is counted as a late collision.
Late collisions are reported by the following 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) 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 that the station reporting the late collision is merely indicating 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.
As discussed before, the maximum number of retries in the backoff algorithm is set to 16. This means that if an interface fails to allocate a slot in which it can transmit its frame without another collision for 16 times, it gives up. The frame is simply not transmitted, and is marked as an excessive collision.
Excessive collisions are reported by the following error messages:
%AMDP2_FE-5-COLL: AMDP2/FE 0/0/[DEC], Excessive collisions, TDR=[DEC], TRC=[DEC] %DEC21140-5-COLL: [chars] excessive collisions %ILACC-5-COLL: Unit [DEC], excessive collisions. TDR=[DEC] %LANCE-5-COLL: Unit [DEC], excessive collisions. TDR=[DEC] %PQUICC-5-COLL: Unit [DEC], excessive collisions. Retry limit [DEC] exceeded %PQUICC_ETHER-5-COLL: Unit [DEC], excessive collisions. Retry limit [DEC] exceeded %PQUICC_FE-5-COLL: PQUICC/FE([DEC]/[DEC]), Excessive collisions, TDR=[DEC], TRC=[DEC] %QUICC_ETHER-5-COLL: Unit [DEC], excessive collisions. Retry limit [DEC] exceeded
The exact error message depends on the platform.
Note: The Transmit Retry Count (TRC) counter is a 4-bit field which indicates the number of transmit retries of the associated packet. The maximum count is fifteen. However, if a Retry Error occurs, the count rolls over to zero. In this case only, the TRC value of zero should be interpreted as meaning sixteen. TRC is written by the controller into the last transmit descriptor of a frame, or when an error terminates a frame.
Note: The time delay reflectometer (TDR) counter is an internal counter that counts the time (in ticks of 100 nanoseconds (ns) each) from the start of a transmission to the occurrence of a collision. Because a transmission travels about 35 feet per tick, this value is useful to determine the approximate distance to a cable fault.
You can check the number of excessive collisions in the output of a show controller ethernet [interface number] command.
Excessive collisions indicate a problem. Common causes are devices connected as full-duplex on a shared Ethernet, broken NICs, or simply too many stations on the shared medium.router#show controller ethernet 0 LANCE unit 0, idb 0xFA6C4, ds 0xFC218, regaddr = 0x2130000, reset_mask 0x2 IB at 0x606E64: mode=0x0000, mcfilter 0000/0000/0100/0000 station address 0010.7b36.1be8 default station address 0010.7b36.1be8 buffer size 1524 RX ring with 16 entries at 0x606EA8 Rxhead = 0x606EC8 (4), Rxp = 0xFC244 (4) 00 pak=0x0FCBF4 Ds=0x60849E status=0x80 max_size=1524 pak_size=66 01 pak=0x10087C Ds=0x6133B6 status=0x80 max_size=1524 pak_size=66 02 pak=0x0FDE94 Ds=0x60BA7E status=0x80 max_size=1524 pak_size=203 03 pak=0x100180 Ds=0x611F82 status=0x80 max_size=1524 pak_size=66 04 pak=0x0FD09C Ds=0x609216 status=0x80 max_size=1524 pak_size=66 05 pak=0x0FE590 Ds=0x60CEB2 status=0x80 max_size=1524 pak_size=66 06 pak=0x100AD0 Ds=0x613A72 status=0x80 max_size=1524 pak_size=66 07 pak=0x0FD9EC Ds=0x60AD06 status=0x80 max_size=1524 pak_size=66 08 pak=0x0FF830 Ds=0x610492 status=0x80 max_size=1524 pak_size=348 09 pak=0x1003D4 Ds=0x61263E status=0x80 max_size=1524 pak_size=343 10 pak=0x0FEA38 Ds=0x60DC2A status=0x80 max_size=1524 pak_size=66 11 pak=0x100D24 Ds=0x61412E status=0x80 max_size=1524 pak_size=64 12 pak=0x0FC74C Ds=0x607726 status=0x80 max_size=1524 pak_size=64 13 pak=0x0FD798 Ds=0x60A64A status=0x80 max_size=1524 pak_size=66 14 pak=0x0FE7E4 Ds=0x60D56E status=0x80 max_size=1524 pak_size=64 15 pak=0x0FD2F0 Ds=0x6098D2 status=0x80 max_size=1524 pak_size=66 TX ring with 4 entries at 0x606F68, tx_count = 0 TX_head = 0x606F80 (3), head_txp = 0xFC294 (3) TX_tail = 0x606F80 (3), tail_txp = 0xFC294 (3) 00 pak=0x000000 Ds=0x63491E status=0x03 status2=0x0000 pak_size=332 01 pak=0x000000 Ds=0x634FDA status=0x03 status2=0x0000 pak_size=327 02 pak=0x000000 Ds=0x630A9E status=0x03 status2=0x0000 pak_size=60 03 pak=0x000000 Ds=0x630A9E status=0x03 status2=0x0000 pak_size=60 3 missed datagrams, 0 overruns 0 transmitter underruns, 0 excessive collisions 8 single collisions, 2 multiple collisions 0 dma memory errors, 0 CRC errors 0 alignment errors, 0 runts, 0 giants 0 tdr, 0 spurious initialization done interrupts 0 no enp status, 0 buffer errors, 0 overflow errors 0 TX_buff, 1 throttled, 1 enabled Lance csr0 = 0x73
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