20-15
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 20 Configuring QoS
Understanding QoS
Queueing and Scheduling on 10/100 Ethernet Ports
Figure 20-6
shows the queueing and scheduling flowchart for 10/100 Ethernet ports.
Figure 20-6 Queueing and Scheduling Flowchart for 10/100 Ethernet Ports
Note
If the expedite queue is enabled, WRR services it until it is empty before servicing the other three
queues.
During the queueing and scheduling process, the switch uses egress queues, which select the
minimum-reserve level and buffer size, and WRR to provide congestion management.
Each 10/100 Ethernet port has four egress queues, one of which can be the egress expedite queue. Each
queue can access one of eight minimum-reserve levels; each level has 100 packets of buffer space by
default for queueing packets. When the buffer specified for the minimum-reserve level is full, packets
are dropped until space is available.
Figure 20-7
is an example of the 10/100 Ethernet port queue assignments, minimum-reserve levels, and
buffer sizes. The figure shows four egress queues per port, with each queue assigned to a
minimum-reserve level. For example, for Fast Ethernet port 0/1, queue 1 is assigned to minimum-reserve
level 1, queue 2 is assigned to minimum-reserve level 3, queue 3 is assigned to minimum-reserve level
5, and queue 4 is assigned to minimum-reserve level 7. You assign the minimum-reserve level to a queue
by using the wrr-queue min-reserve interface configuration command.
65128
No
Yes
Queue number
Read the CoS value of
CoS-to-queue map.
Get minimum-reserve level
and queue size.
Is space
available?
Put packet into specified
queue and service queue
according to WRR.
Drop packets until
space is available.
Start
Done
