3Com Switch 8800 Configuration Guide Chapter 33 QoS Configuration
33-3
Figure 33-2 Ethernet frame with 802.1Q tag header
In the above figure, each host supporting 802.1Q protocol adds a 4-byte 802.1Q tag
header after the source address in Ethernet header.
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 33-3.
Figure 33-3 802.1Q 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.
VII. 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