Allied Telesis AT-S63 - Manual (Page 165)

AT-S63 Management Software Features Guide
Section II: Advanced Operations 165
Both the VLAN tag User Priority and the traffic class / flow group priority
setting allow eight different priority values (0-7). These eight priorities are
mapped to the switch’s eight CoS queues. The switch’s default mapping is
shown in Table 48 on page 150. 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.