Allied Telesis AT-S62 - Manual (Page 258)

Chapter 16: Quality of Service
Section II: Advanced Operations 258
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 four Class of Service (CoS) egress queues, numbered from
0 to 3. Queue 3 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 sufficiently over-subscribed so that egress queues are
always full. If one type of traffic is causing the congestion, you can limit
its bandwidth. Other solutions in this situation 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 four CoS queues. The switch’s default
mapping is shown in Table 7 on page 290. 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 this 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:
❑ 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