Allied Telesis AT-S63 - Manual (Page 342)

Chapter 17: Quality of Service
342 Section II: Advanced Operations
in the appropriate CoS queue for its VLAN tag User Priority field. If neither
the traffic class / flow group priority nor the VLAN tag User Priority is set,
the packet is sent to the default queue, queue 1.
Both the VLAN tag User Priority and the traffic class / flow group priority
setting allow eight different priority values (0-7). These eight priorities are
mapped to the switch’s eight CoS queues. The switch’s default mapping is
shown in Table 7 on page 323. Note that priority 0 is mapped to CoS
queue 1 instead of CoS queue 0 because tagged traffic that has never
been prioritized has a VLAN tag User Priority of 0. If priority 0 was mapped
to CoS queue 0, this default traffic goes to the lowest queue, which is
probably undesirable. This mapping also makes it possible to give some
traffic a lower priority than the default traffic.
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. Replacing this field is most useful outside
DiffServ domains.
ˆ The DSCP value of the IP header’s TOS byte (Figure 86 on page 287).
Replacing this field may be required as part of the configuration of a
DiffServ domain. See “DiffServ Domains” on page 343 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.