Juniper Networks EX2500 - Manual (Page 79)

Using 802.1p Priority to Provide QoS „ 65
Chapter 5: Quality of Service
Using 802.1p Priority to Provide QoS
The EX2500 switch provides Quality of Service (QoS) 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 through 7). Priority 7 is
assigned to highest-priority network traffic, such as OSPF or RIP routing table
updates, priorities 5 through 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 13 shows the priority bits in a VLAN-tagged packet.
Figure 13: Layer 2 802.1q/802.1p VLAN-Tagged Packet
Ingress packets receive a priority value, as follows:
„ Tagged packets—The switch reads the 802.1p priority in the VLAN tag.
„ Untagged packets—The 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 Class of Service (COS) queue based on the priority
value, and scheduled for transmission based on the COS queue number. Higher
COS queue numbers provide forwarding precedence.
The following is an example of 802.1p configuration:
1. Configure a port’s default 802.1p priority value to 2.
ex2500(config)# interface port 1
ex2500(config-if)# dot1p 2
ex2500(config-if)# exit
2. Map the 802.1p priority value to a COS queue.
ex2500(config)# qos transmit-queue mapping 1 0
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