140 RackSwitch G8000: Application Guide
Using 802.1p Priority to Provide QoS
The G8000 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 to be given to packets during forwarding. Packets with a numerically
higher (non-zero) priority are given forwarding preference over packets with lower
priority value.
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.
Figure 17. 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
value, based on the port’s default 802.1p priority.
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.
To configure a port’s default 802.1p priority value, use the following commands.
RS G8000(config)# interface port 1
RS G8000(config-if)# dot1p <802.1p value (0-7)>
RS G8000(config-if)# exit
7 6 5 4 3 2 1 0
Priority
7 6 5 4 3 2 1 0
VLAN Identifier (VID)
SFD
DMAC
SMAC Tag
E Type
Data
FCS
Preamble
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