
Quality of Service (QoS)
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
39.3
QoS Packet Information
Provision for QoS information to be embedded within the data fields exists within both the
data link and network layer protocols. This information can then be used to assess the priority
of the data and the resource preferences that need to be applied. The process of applying
these service quality tags to your data is known as marking.
Link Layer QOS
Link layer frames entering a port may either be tagged or untagged. VLAN tagged frames
contain the additional 802.1Q tag fields shown in
Figure 39-1 on page 39.3
below. Located
within the TCI is a three bit User Priority field. This field is specifically provided to attach QoS
based priority information, often referred to as the Class of Service (CoS) field.
Figure 39-1: IEEE 802.1Q Tagging
Appendix G of the IEEE Standard 802.1D provides some useful guidelines on applying priorities
to 7 traffic types. These are summarized in the
Table 39-1
below:
Table 39-1: CoS Traffic Mapping Guidelines
On the switch you can use the command,
“match cos” on page 40.10
to select frames that
match a particular User Priority value and assign them to a particular class-map. You can then
map these incoming frames to one of eight egress queues. This facility enables you to accept
frames that are already carrying meaningful priority information and automatically assign them
to an appropriate egress queue. For example, you could decide to send frames with a User
Priority value of 7 to queue 3, and frames with a User Priority value of 2 to queue 7. The
process of assigning queues based on CoS tags is commonly known as “PreMarking”
Note:
You configure the pre-marking steps to an ingress port. This process marks the data
packets so that when they reach the egress port the decisions made during pre-
marking can be applied in accordance with the configuration of the egress port.
User
Priority
Traffic Types
1
Background
2
Spare
0
Best Effort
3
Excellent Effort
4
Controlled Load
5
Video <100 ms latency and jitter
6
Voice <10 ms latency and jitter
7
Network Control
VLAN_UPriority
C
F
I
VLAN I D
User
Priority