3Com 4800G Series, Configuration Manual

Page: 864 / 1246

864 C
HAPTER
66: T
RAFFIC
C
LASSIFICATION
, TP,
AND
LR C
ONFIGURATION
Figure 258 802.1Q tag headers
In the figure above, the 3-bit priority field in TCI is 802.1p precedence in the range
of 0 to 7. In the figure above, the priority field (three bits in length) in TCI is
802.1p precedence (also known as CoS precedence), which ranges from 0 to 7.
The precedence is called 802.1p precedence because the related applications of
this precedence are defined in detail in the 802.1p specifications.
TP and LR Overview
If the traffic from users is not limited, a large amount of continuous burst packets
will result in worse network congestion. The traffic of users must be limited in
order to make better use of the limited network resources and provide better
service for more users. For example, if a traffic flow obtains only the resources
committed to it within a certain period of time, network congestion due to
excessive burst traffic can be avoided.
TP is traffic control policies for limiting traffic and resource usage by supervising
the traffic. The prerequisite for TP is to determine whether or not the traffic
exceeds the set threshold. Traffic control policies are adopted only when the traffic
exceeds the set threshold. Generally, token bucket is used for evaluating traffic.
Traffic Evaluation and
Token Bucket
Token Bucket A token bucket can be considered as a container with a certain capacity to hold
tokens. The system puts tokens into the bucket at a pre-set rate. When the token
bucket is full, the extra tokens will overflow and the number of tokens in the
bucket stops increasing.
Table 67 Description on 802.1p precedence
802.1p precedence
(decimal) 802.1p
precedence (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)
Priority VLAN ID
Byte 2 Byte 3 Byte 4
0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0
3 2 7 4 1 0 6 5 3 2 7 4 1 0 6 5 3 2 7 4 1 0 6 5 3 2 7 4 1 0 6 5
cfi
TCI (Tag Control Information)