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

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

Using 802.1p Priority to Provide QoS „ 63

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Juniper Networks EX2500 manual Using 802.1p Priority to Provide QoS, Shows the priority bits in a VLAN-tagged packet