Allied Telesis AT-S63 - Manual (Page 340)

Chapter 17: Quality of Service
340 Section II: Advanced Operations
ˆ Replacing the TOS precedence or DSCP value to enable the next
switch in the network to process the packet correctly.
Bandwidth
Allocation
Bandwidth limiting is configured at the level of traffic classes, and
encompasses the flow groups contained in the traffic class. Traffic classes
can be assigned maximum bandwidths, specified in kbps, Mbps, or Gbps.
Packet
Prioritization
The switch has eight Class of Service (CoS) egress queues, numbered
from 0 to 7. Queue 7 has the highest priority. When the switch becomes
congested, it gives high priority queues precedence over lower-priority
queues. When the switch has information about a packet’s priority, it
sends the packet to the appropriate queue. You can specify the queue
where the switch sends traffic, how much precedence each queue has,
and whether priority remapping is written into the packet’s header for the
next hop to use.
Prioritizing packets cannot improve your network’s performance when
bandwidth is over-subscribed to the point that egress queues are always
full. If one type of traffic is causing the congestion, you can limit its
bandwidth. Other solutions are to increase bandwidth or decrease traffic.
You can set a packet’s priority by configuring a priority in the flow group or
traffic class to which the packet belongs. The packet is put in the
appropriate CoS queue for that priority. If the flow group and traffic class
do not include a priority, the switch can determine the priority from the
VLAN tag User Priority field of incoming tagged packets. The packet is put
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 8 on page 321. 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.