Allied Telesis AT-S25 - Manual (Page 162)

Section II: Local and Telnet Mangement
162
Class of Service Overview
The AT-8316F or AT-8324 Switch supports CoS as specified in the IEEE
802.1p and 802.1Q standards. CoS can be important in network
environments where there are time-critical applications, such as voice
transmission or video conferencing, that can be adversely affected by
packet transfer delays.
Prior to CoS, network traffic was handled in a best-effort manner. File
transfer delays did occur, but were mostly transparent to network users.
But with the introduction of time-critical applications, packet transfer
delays can prove problematic. For example, transfer delays of voice
transmission can result in poor audio quality.
CoS was designed to address this problem. The 802.1p standard outlines
eight levels of priority, 0 to 7, with 0 the lowest priority and 7 the highest.
The AT-8316F or AT-8324 Switch has two priority queues, low and high.
When a priority tagged packet enters a switch port, the switch responds
by placing the packet into one of the two queues according to following
behaviors:
❑ For all IGMP and BPDU packets that are destined to the CPU, the
switch sends these packets to “high” queue.
❑ For all incoming unicast packets that have unknown destination
on the port, the switch sends these packets to “low” queue.
❑ For all other port incoming packets, the switch responds as follow.
Note
These priority-to-queue assignments can be overridden using the
AT-S25 management software on a per port basis.
Override
Port Priority
Level
Tag Priority
Level
Priority Queue
1
1. To send a packet to the low queue, one of the following conditions must exist:
* Receive a packet with an unknown destination address.
* Set the override to “yes” and the port priority to “0”.
* Set the override to “no”, the port priority to “0”, and receive an “untagged” packet or a packet with a priority tag “less than 4”.
- 1 - high
yes 0 - low
no 0 > 3 high
no 0 < 4
(or untagged)
low