3Com 7757 Configuration Manual page 660

660 C 61: Q S C
HAPTER O ONFIGURATION
Priority of Protocol
Packets
Priority Remark
Packet Filter
As shown in the figure above, each host supporting 802.1Q protocol adds a 4-bit
802.1Q tag header after the source address of the former Ethernet frame header
when sending packets.
The 4-bit 802.1Q tag header contains a 2-bit Tag Protocol Identifier (TPID) whose
value is 8100 and a 2-bit Tag Control Information (TCI). TPID is a new class defined
by IEEE to indicate a packet with an 802.1Q tag. Figure 172 describes the detailed
contents of an 802.1Q tag header.
Figure 172 802.1Q tag headers
Byte 1
TPID (Tag Protocol Identifier)
1 0 0 0 0 0 0 1
7 6 5 4 3 2 1 0
In the figure above, the 3-bit priority field in TCI is 802.1p priority in the range of 0
to 7. The 3 bits specify the precedence of the frame. 8 classes of precedence are
used to determine which packet is sent preferentially when the switch is
congested.
Table 530 Description on 802.1p priority
CoS (decimal)
0
1
2
3
4
5
6
7
The precedence is called 802.1p priority because the related applications of this
precedence are defined in detail in the 802.1p specification.
3 Local precedence
Local precedence is the precedence of an outbound queue on a port of the switch.
It is in the range of 0 to 7. Each outbound queue has its own local precedence.
Protocol packets carry their own priority. You can perform QoS actions on protocol
packets by setting their priorities.
The priority remark function is to use ACL rules in traffic identification and remark
the priority for the packets matching with the ACL rules.
Packet filter means filtering the service traffic. For example, in the operation of
dropping packets, the service traffic matching with the traffic classification rule is
Byte 2
0 0 0 0 0 0 0 0 Priority
7 6 5 4 3 2 1 0 7
CoS (binary)
000
001
010
011
100
101
110
111
Byte 3
TCI (Tag Control Information)
cfi VLAN ID
6 5 4 3 2 1 0 7 6 5
Description
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management
Byte 4
4 3 2 1 0