RackSwitch G8000 Application Guide
Chapter 6: Quality of Service
111BMD00041, November 2008
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 that should 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 6-3 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 trans-
mission based on the scheduling weight of the COS queue.
Priority
7 6 5 4 3 2 1 0
VLAN Identifier (VID)
SFD
DMAC
SMAC Tag
E Type
Data
FCS
Preamble
