3Com 4007, Implementation Manual, Page: 204 / 684

204 C
HAPTER
10: C
LASS OF
S
ERVICE
(C
O
S)
Important Considerations
■
In non-blocking situations, CoS settings have no impact on traffic flow
through the module.
■
In blocking situations, queue 1 (high priority) traffic on a given port is
processed ahead of queue 2 traffic on that same port. Traffic in
queue 2 on that port is either delayed (buffered) or dropped (if buffers
become full) as needed to allow the queue 1 traffic to be forwarded.
■
Queue 1 traffic on a given port is not necessarily processed ahead of
queue 2 traffic from other ports. This is because the switch selects
traffic from each port in an approximate round-robin fashion.
Configuring Priority
Levels
By default, CoS is enabled with priorities 4,5,6, and 7 assigned to
queue 1 and priorities 0,1, 2, and 3 assigned to queue 2. This
arrangement conforms with IEEE recommendations, but you can change
it at any time.
Although there are two physical queues per port, the priority levels (traffic
classes) that you assign to each queue actually apply to all ports in the
module.
When you assign one or more priority levels to one of the queues, the
module automatically assigns the remaining priority levels to the other
queue.
If CoS is disabled, you can still modify the priority assignments to each
queue; they simply do not effect traffic until you enable CoS.
Configuring a Rate
Limit on Queue 1
You can configure a rate limit for queue 1. The rate limit is configured as
a percentage of the number of packets received on each port.
The percentage refers to the number of packets that are processed from
queue 1 out of every 8 packets received on the port. This
n of 8 packets
formula means that, in real terms, there are eight supported rate limit
percentages: 12.5, 25, 37.5, 50, 62.5, 75, 87.5, and 100. The rate limit
operates as a threshold, not as a bandwidth reservation technique.
Considering that the module does not accept decimal values, you must
enter a whole number in the range that corresponds to one of the eight
percentages. Depending on the number you enter, the module rounds
down to the nearest
n of 8
value, although the summary display retains
the number that you enter.