Blade ICE G8000 Using 802.1p Priority to Provide QoS, 3Layer 2 802.1q/802.1p Vlan tagged packet

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 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.

Preamble

SFD

DMAC SMAC

Tag

E Type Data

FCS

	Priority							VLAN		Identifier		(VID)
7	6	5	4	3	2	1	0	7	6		5		4	3	2	1	0

Figure 6-3Layer 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.

BMD00041, November 2008 Chapter 6: Quality of Service „ 111