3Com 8807 Configuration Manual page 183

The 802.1Q tag header contains a 2-byte TPID (Tag protocol Identifier, with the
value 8100) and a 2-byte TCI (tag control information). TPID is newly defined by
IEEE to represent a packet with 802.1Q tag added. The contents of 802.1Q tag
header are shown in Figure 44.
Figure 44 802.1Q tag header
In the figure, the priority field in TCI stands for 802.1p priority, which consists of
three bits. There are eight priority levels, numbered as 0 to 7, for determining to
send which packets first when switch congestion takes place.
Since their applications are defined in detail in the 802.1p Recommendation, they
are named as 802.1p priority levels.
Queue scheduling
Queue scheduling is used to resolve problems of resource contention by many
packets. These algorithms are often used in queue scheduling: strict priority (SP)
algorithm and weighted round Robin (WRR) algorithm.
1 SP algorithm
Figure 45 Priority queues
high priority
Packets sent via this
interface
Classify
Low priority
SP algorithm is designed for key services. One of the characteristics of key services
is these services should be processed first to minimize response delay during
switch congestion. For example, there are eight outbound queues at the port,
numbered respectively as 7 to 0, with priority levels in descending order.
In SP mode, the system first sends those packets of higher priority in strict
accordance with priority order. Only when packets in high priority queue are all
QoS Overview
queue 7
queue 6
queue 5
queue 4
Dequeue Sending queue
queue 3
queue 2
queue 1
queue 0
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