3com 8807 사용자 가이드

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 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

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 

Packets sent via this

interface

high priority

Low priority

Classify

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Sending queue

Dequeue

queue 7

queue 6

queue 5

queue 4

queue 3

queue 2

queue 1

queue 0