Handbook

N8406-023 1Gb Intelligent L3 Switch Application Guide 84
Table 19 Class selector priority classes
Priority
Class Selector
DSCP
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 20 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 switch has two
output Class of Service queues (COSq). The scheduling scheme is Weight Round Robin (WRR), with user-
configurable weight from 1 to 15 for a COSq. The weight of 0 (zero) indicates strict priority, which might starve the
another queue.
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.
Figure 13 Layer 2 802.1q/802.1p VLAN tagged packet
Ingress packets receive a priority value, as follows:
Tagged packets—switch reads the 802.1p priority in the VLAN tag.
Untagged packets—switch tags the packet and assigns an 802.1p priority, based on the port‘s default priority
(/cfg/port x/8021ppri).
Egress packets are placed in a COS queue based on the priority value, and scheduled for transmission based on
the scheduling weight of the COS queue.