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Chapter 13: Class of Service

150 Section II: Advanced Operations

For example, when a tagged packet with a priority level of 3 enters a port

on the switch, the packet is stored in Q3 queue on the egress port.

Note that priority 0 is mapped to CoS queue 1 instead of CoS queue 0

because tagged traffic that has never been prioritized has a VLAN tag

User Priority of 0. If priority 0 was mapped to CoS queue 0, this default

traffic goes to the lowest queue, which is probably undesirable. This

mapping also makes it possible to give some traffic a lower priority than

the default traffic.

You can change these mappings. For example, you might decide that

packets with a priority of 5 should be handled by egress queue Q3 and

packets with a priority of 2 should be handled in Q1. The result is shown in

Table 49.

Table 48. Default Mappings of IEEE 802.1p Priority Levels to Priority

Queues

IEEE 802.1p Priority

Level

Port Priority Queue

0 Q1

1 Q0 (lowest)

2Q2

3Q3

4Q4

5Q5

6Q6

7 Q7 (highest)

Table 49. Customized Mappings of IEEE 802.1p Priority Levels to Priority