QoS Supported By Switch 4210 Family 303
The 4-byte 802.1Q tag header consists of the tag protocol identifier (TPID, two
bytes in length), whose value is 0x8100, and the tag control information (TCI, two
bytes in length).
Figure 94 describes the detailed contents of an 802.1Q tag
header.
Figure 94 802.1Q tag headers
In the figure above, the priority field (three bits in length) in TCI is 802.1p priority
(also known as CoS precedence), which ranges from 0 to 7.
The precedence is called 802.1p priority because the related applications of this
precedence are defined in detail in the 802.1p specifications.
Priority Trust Mode After a packet enters a switch, the switch sets the 802.1p priority and local
precedence for the packet according to its own capability and the corresponding
rules. The local precedence is locally significant precedence that the switch assigns
to the packet. It corresponds to an output queue. Packets with higher local
precedence values take precedence over those with lower precedence values and
will be processed preferentially.
By default, a Switch 4210 processes a received packet as follows:
■ For a packet without an 802.1q tag header, the switch uses the priority of the
receiving port as the 802.1p precedence of the packet and looks up it in the
802.1p-precedence-to-local-precedence mapping table for the local
precedence, and then assigns the local precedence to the packet for it to be
added to a output queue.
■ For a packet with an 802.1q tag header, the switch replaces the 802.1p
precedence of the packet with the priority of the receiving port and looks up
the latter in the 802.1p-precedence-to-local-precedence mapping table for the
local precedence, and then assigns the local precedence to the packet for it to
be added to an output queue.
Tab le 222 Description of 802.1p priority
802.1p priority (decimal) 802.1p priority (binary) Description
0 000 best-effort
1 001 background
2 010 spare
3 011 excellent-effort
4 100 controlled-load
5 101 video
6 110 voice
7 111 network-management
Byte 1
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
Priority cfi VLAN ID
Byte 2Byte 3Byte 4
00 0 00 0 001 0 0 00 10 0
3274 1065 3 274 1065 3274 1065 3274 1065
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