Allied Telesis AT-S63 - Manual (Page 314)

Chapter 15: Class of Service
314 Section II: Advanced Operations
The procedure for changing the default mappings is found in “Mapping
CoS Priorities to Egress Queues” on page 320. 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 instead use a temporary priority level assigned to the
port. For instance, perhaps you decide that all tagged packets received on
port 4 should be assigned a priority level of 5, regardless of the priority
level in the packets themselves. The procedure for overriding priority
levels is explained in “Configuring CoS” on page 317.
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 assigned a priority of 0 and are placed in a port’s Q1
egress queue. But you can override this and instruct a port’s untagged
frames to be stored in a different priority queue. The procedure for this is
also explained in “Configuring CoS” on page 317.
One last thing to note is that CoS 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 eight egress queues. For example, if all the
queues contain packets, should the port transmit all packets from Q7, 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 Q6
7 Q7 (highest)
Table 12. Customized Mappings of IEEE 802.1p Priority Levels to Priority
Queues (Continued)
IEEE 802.1p Priority
Level
Port Priority Queue