Collision domain problems affect the local medium, and disrupt
communications to Layer 2 or Layer 3 infrastructure devices, local servers or
services. They typically result from the following problems:

- Bad cables
- Marginal or intermittent workstation NICs
- Marginal or intermittent ports on hubs or switches
- Errors or excessive traffic on the local collision domain
- Duplex mismatches
- Electrical noise and other environmental disruptions
Collisions are normally a more significant problem on shared media than
on switch ports. Average collision counts on shared media should generally be
below 5 percent, though that number is conservative. Be sure that judgments are
based on the average and not a peak or spike in collisions.
If the
average utilization is high (sustained peaks in excess of 60 percent for shared
media, and in excess of 80-90 percent for switched links) and collision counts
are acceptable (average is below 5 percent for shared media, and below 1
percent for switched links), then the network may simply be saturated. There
may be too many stations transmitting within this collision domain, or the
network architecture may need optimizing for shorter distances between distant
stations.
Be careful when troubleshooting collision problems because the
obvious answer is usually wrong. The addresses found in collision fragments
belong to stations that transmitted legally. Stations that sent enough of the
current frame to have a source address in a collision fragment usually started
transmitting first, though that depends in part on the monitoring point within
the collision domain. Most of the stations that collide with those legally
transmitted frames are also operating legally. They did not "hear"
anything on the wire, so they began to transmit. If there is a station that has
gone "deaf" and is stepping on other transmissions because it does
not hear them, it will probably never be discovered because its frame collides
with another transmission and its data is always corrupted. Troubleshoot the
presence of too many collisions, but don’t examine the fragments closely. Using
the corrupted data from collision fragments will just cause frustration.
Late Collisions
A late collision is counted when a collision is
detected by a device after it has sent the 512th bit of its frame. No more than
a few late collisions should ever occur in any environment. If a device is
incrementing a collision counter, further investigation is needed as a
significant problem is occurring. If the number of late collisions is occurring
at a steady rate, performance degradation may be noted. Any of the following
conditions may be causing late collisions:
- Incorrect configuration
- Duplex mismatch (one host operating at half-duplex while another host is
operating a full-duplex)
- Faulty cabling
- Faulty hub or shared media device
- Faulty NIC or switch port
- Excessive network traffic beyond the limitations of the shared media hub or
switch port
Note: Collisions and late collisions should never occur on any router,
switch, or NIC port operating a full-duplex. Some Cisco Catalyst switches, such
as the Catalyst 6000/6500, will disable ports on excessive late collisions,
even when operating a half-duplex.