Allied Telesis AT-S63 - Manual (Page 340)

Chapter 17: Class of Service
340 Section II: Advanced Operations
The procedure for changing the default mappings is found in “Mapping
CoS Priorities to Egress Queues” on page 346. Note that because all
ports must use the same priority-to-egress queue mappings, these
mappings are applied at the switch level. They cannot be set on a per-port
basis.
You can configure a port to completely ignore the priority levels in its
tagged packets and store all the packets in the same egress queue. For
instance, perhaps you decide that all tagged packets received on port 4
should be stored in an egress port’s Q3 egress queue, regardless of the
priority level in the packets themselves. The procedure for overriding
priority levels is explained in “Configuring CoS” on page 343.
CoS relates primarily to tagged packets rather than untagged packets
because untagged packets do not contain a priority level. By default, all
untagged packets are placed in a port’s Q0 egress queue, the queue with
the lowest priority. But you can override this and instruct a port’s untagged
frames to be stored in a higher priority queue. The procedure for this is
also explained in “Configuring CoS” on page 343.
One last thing to note is that the AT-S63 software does not change the
priority level in a tagged packet. The packet leaves the switch with the
same priority it had when it entered. This is true even if you change the
default priority-to-egress queue mappings.
Scheduling
A switch port needs a mechanism for knowing the order in which it should
handle the packets in its four egress queues. For example, if all the
queues contain packets, should the port transmit all packets from Q3, the
highest priority queue, before moving on to the other queues, or should it
instead just do a few packets from each queue and, if so, how many?
This control mechanism is called scheduling. Scheduling determines the
order in which a port handles the packets in its egress queues. The
AT-S63 software has two types of scheduling:
ˆ Strict priority
ˆ Weighted round robin priority
5 Q3
6 Q3
7 Q3
Table 7. Customized Mappings of IEEE 802.1p Priority Levels to Priority
Queues
IEEE 802.1p Priority
Level
Port Priority Queue