How Traffic Prioritization Works 53
appropriate egress port(s). When the packet reaches the head of its
queue and is about to be transmitted the device determines whether or
not the egress port is tagged for that VLAN. If it is, then the new 802.1p
tag is used in the extended 802.1D header.
The IEEE 802.1D standard specifies eight distinct levels of priority (0 to 7),
each of which relates to a particular type of traffic. The priority levels and
their traffic types are shown in Figure 14
in order of increasing priority.
The mapping from 802.1p level to traffic queue in the Switch is
proprietary and is slightly different to the recommended IEEE mapping.
Figure 14 IEEE 802.1p priority levels and recommended IEEE 802.1D traffic
types
The number of queues and their mappings to the 8 levels is proprietary
and can even vary between Switches from the same vendor.
You cannot alter the mapping between the IEEE 802.1p priorities and the
traffic queues. These are calculated to be the most efficient, and are fixed
as illustrated in Figure 14
.
Queue 1 Low Priority()
Ingress Port
Queue 7
Traffic Queues
Queue 2
Queue 5 (Priority)
Network Control 7
Voice
6
Video
5
Multimedia
4
Business Critical
3
Best Effort
0
802.1D
Switch 3812 and Switch 3824
Egress Port
802.1p
Service Levels
Traffic
Classification
Queue 3
Queue 4
Queue 6
Queue 8 (High Priority)
Spare
2
Background
1
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