© Copyright Lenovo 2016 Chapter 13: Quality of Service 245Using 802.1p Priority to Provide QoSThe G8264 provides Quality of Service functions based on the priority bits in apacket’s VLAN header. (The priority bits are defined by the 802.1p standard withinthe IEEE 802.1Q VLAN header.) The 802.1p bits, if present in the packet, specify thepriority to be given to packets during forwarding. Packets with a numericallyhigher (non‐zero) priority are given forwarding preference over packets withlower priority value.The IEEE 802.1p standard uses eight levels of priority (0‐7). Priority 7 is assigned tohighest 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 trafficpriority has not been configured on your network. The switch can filter packetsbased on the 802.1p values.Figure 24. Layer 2 802.1q/802.1p VLAN tagged packetIngress 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, andscheduled for transmission based on the COS queue number. Higher COS queuenumbers provide forwarding precedenceTo configure a port’s default 802.1p priority value, use the following commands:RS G8264(config)# interface port 1RS G8264(config if)# dot1p <802.1p value (0‐7)>RS G8264(config if)# exit7 6 5 4 3 2 1 0Priority7 6 5 4 3 2 1 0VLAN Identifier (VID)SFD DMAC SMAC Tag E Type Data FCSPreamble
