QoS Configuration
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20.1.3 Priority
(1) 802.1p priority lies in Layer 2 packet headers and is applicable to occasions where the
Layer 3 packet header does not need analysis but QoS must be assured at Layer 2. As
shown in the chapter of VLAN configuration. Each host supported 802.1Q protocol forwards
packets which are from Ethernet frame source address add a 4-byte tag header. Figure as
20-1.
Figure 20-1 802.1Q tag
As shown in the figure above, PRI segment is 802.1p priority. It consists of 3bits whose
range from 0~7. The three bits point the frame priority. The tag including 8 formats gives the
precedence to forward the packets.
Table 20-1 Description on 802.1Q values
Cos
(
decimal
)
Cos (binary
)
Descrption
0
000
spare
1
001
background
2
010
best-effort
3
011
excellent-effort
4
100
controlled-load
5
101
video
6
110
voice
7
111
network-management
(2)IP precedence, TOS precedence, and DSCP values
The TOS field in the IP header contains eight bits: the first three bits represent IP
precedence; the subsequent four bits represent a ToS value and 1 bit with currently unused
defaults 0. The four bits of TOS packets are grouped into four classes: the smallest time delay,
maximum rate, highly reliability, minimum cost. Only 1 bit can be set, if the DSCP values
equal 0, that means normal service.
Figure 20-2 IP precedence and TOS precedence
Summary of Contents for S6424-S2C2 series
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