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.
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.
