3com 8807 사용자 가이드

C

HAPTER

 22: Q

O

S C

ONFIGURATION

Step 2: Run filtering operation (deny or permit) to the identified traffic. By default, 
permit operation is selected.

QoS can police traffic at the ingress port, to provide better services with the limited 
network resources.

You can re-specify forwarding direction for packets, based on QoS policy.

Ethernet switches can provide priority tags, including ToS, DSCP, 802.1p, and so 
on, for specific packets. These priority tags are applicable to different QoS models.

The following describes IP priority, ToS priority, DCSP priority, Exp priority and 
802.1p priority.

1 IP priority, ToS priority, DSCP priority and Exp priority 

Figure 42   DS field and ToS byte

As shown in Figure 42, the ToS field in the IP header contains 8 bits. The first three 
bits represent IP priority, in the range of 0 to 7; bits 3-6 stand for ToS priority, in the 
range of 0 to 15. RFC2474 redefines the ToS field in IP packets as DS 
(differentiated services) field. The first six bits denote DSCP (differentiated services 
codepoint) priority, in the range of 0 to 63, the latter two bits are reserved. The 
first three bits (bit 0~2) of DSCP priority represent Exp priority, in the range of 0 to 
7.

2 802.1p priority

802.1p priority is stored in the header of Layer 2 packets and is suitable for the 
case where only Layer 2 QoS guarantee, not L3 header analysis, is required.

Figure 43   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.