Content

| 3.2 |  | Characteristics of Physical Layer Optimization Problems |
| 3.2.4 |  | Collisions |

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.

Physical layer or Layer 1 network devices act to increase collision domains, effectively increasing the scope of a potential problem from bad to worse. Layer 1 devices such as repeaters and hubs act to regenerate signals and therefore increase the maximum distance over which a network can operate. Hubs additionally act to increase the collision domain. They contain from four to forty-eight ports for station connectivity. A hub will regenerate any signal received and resend it out all active ports.

Be very 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.

The old attribution that a late collision is literally the result of a too-long cable is challenged by calculating the delay introduced by the cable only. While delay introduced by the cable is a factor, it is much more likely that other factors such as cable impedance mismatches, signal attenuation along the cable, too many repeaters, and marginal interfaces result in situations where a shortened cable eliminates late collisions.