Collisions may occour on ethernet links which are operating at Half Duplex (HDX). They do not occour on links using Full Duplex (FDX). The normal formula to calculate the packet collision rate is:

( collisions / output packets ) * 100 = collision rate

For example:

# netstat -i 10

input   lo0:1     output           input  (Total)    output
packets errs  packets errs  colls  packets errs  packets errs  colls
135299  1     19240   0     691    135910  1     19851   0     691

# bc -l

691/19240 * 100 <RETURN>
3.59147609147609147600 (= 3.59 % collision rate)

The following rough guidelines are common among network administrators.

• Collision rates < 5% are considered ok.
• Collision rates > 5% are the first indication of possible network loading
• Collision rates > 10% may signify an over-loaded network.

It is important to note that the collision rate:

1. of a given machine does not represent the average for the entire network,
2. threshold varies greatly, and
3. it is not an indication of actual load on the network (Collisions + Defers).

NOTE 1 : When looking at a systems collisions compared to output packets it is important to note that this is only one node’s perspective of the network while it is transmitting.

NOTE 2 : Depending on the applications running and protocol used, a high collision rate (>50%) may not cause a decrease in perceived throughput on a client, but a server may show performance degradation at 5-10%.

NOTE 3 : It is recommended to use a Network Sniffer/Analyzer to get a overall network analysis and better identify the problem.

NOTE 4 : Segmentation and/or the use of ethernet switches is normally the solution to a network-wide collision and netload issue.





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