Allied Telesis AT-S63 - Manual (Page 166)

Chapter 14: Quality of Service
166 Section II: Advanced Operations
Replacing Priorities
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 6 on page 131).
Replacing this field may be required as part of the configuration of a
DiffServ domain. See “DiffServ Domains” on page 167 for information
on using the QoS policy model and the DSCP value to configure a
DiffServ domain.
VLAN Tag User Priorities
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.
DSCP Values
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.