Allied Telesis AT-S63 User Manual

Chapter 13: Quality of Service

150

Section II: Advanced Operations

The traffic class or flow group priority (if set) determines the egress queue 
a packet is sent to when it egresses the switch, but by default has no 
effect on how the rest of the network processes the packet. To 
permanently change the packet’s priority, you need to replace one of two 
priority fields in the packet header:

ˆ

The User Priority field of the VLAN tag header. Replacing this field 
relabels VLAN-tagged traffic, so that downstream switches can 
process it appropriately. 

ˆ

The DSCP value of the IP header’s TOS byte (Figure 5 on page 115). 
Replacing this field may be required as part of the configuration of a 
DiffServ domain. See “DiffServ Domains” on page 151 for information 
on using the QoS policy model and the DSCP value to configure a 
DiffServ domain.

Within a flow group or traffic class, the VLAN tag User Priority value of 
incoming packets can be replaced with the priority specified in the flow 
group or traffic class. Replacement occurs before the packet is queued, so 
this priority also sets the queue priority.

There are three methods for replacing the DSCP byte of an incoming 
packet. You can use these methods together or separately. They are 
described in the order in which the switch performs them.

ˆ

The DSCP value can be overwritten at ingress, for all traffic in a policy.

ˆ

The DSCP value in the packet can be replaced at the traffic class or 
flow group level.

ˆ

You can use these two replacements together at the edge of a DiffServ 
domain, to initialize incoming traffic.

ˆ

The DSCP value in a flow of packets can replaced if the bandwidth 
allocated to that traffic class is exceeded. This option allows the next 
switch in the network to identify traffic that exceeded the bandwidth 
allocation.