Handbook
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Class Selector (CS) : This PHB has 8 priority classes, with CS7 representing the highest priority, and CS0
representing the lowest priority, as shown below. CS PHB is described in RFC 2474.
Table 18 Class selector priority classes
Priority
Class Selector
DSCP
Highest
CS7
56
CS6
48
CS5
40
CS4
32
CS3
24
CS2
16
CS1
8
Lowest
CS0
0
QoS levels
The following table shows the default service levels provided by the switch, listed from highest to lowest
importance:
Table 19 Default QoS service levels
Service Level
Default PHB
802.1p Priority
Critical
CS7
7
Network Control
CS6
6
Premium
EF, CS5
5
Platinum
AF41, AF42, AF43, CS4
4
Gold
AF31, AF32, AF33, CS3
3
Silver
AF21, AF22, AF23, CS2
2
Bronze
AF11, AF12, AF13, CS1
1
Using 802.1p priorities to provide QoS
The switch provides Quality of Service functions based on the priority bits in a packet‘s VLAN header. (The
priority bits are defined by the 802.1p standard within the IEEE 802.1q VLAN header.) The 802.1p bits, if
present in the packet, specify the priority given to packets during forwarding. Packets with a numerically
higher (non-zero) priority are given forwarding preference over packets with lower priority.
Packets with a priority mapped to a higher Class of Service (COS) and COS queue (COSq) weight are
given forwarding preference over packets with priority mapped to a lower COS and COSq weight. The
scheduling scheme is Weight Round Robin (WRR), with user-configurable weight from 1 to 15 for a COSq.
The switch can be configured with either two or eight output Class of Service queues (COSq)..
The IEEE 802.1p standard uses eight levels of priority (0-7). Priority 7 is assigned to highest priority network
traffic, such as OSPF or RIP routing table updates, priorities 5-6 are assigned to delay-sensitive applications
such as voice and video, and lower priorities are assigned to standard applications. A value of 0 (zero)
indicates a ―best effort‖ traffic prioritization, and this is the default when traffic priority has not been
configured on your network. The switch can filter packets based on the 802.1p values, and it can assign
or overwrite the 802.1p value in the packet.