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ONFIGURATION
5-33
QOS Settings
QoS (Quality of Service) is a mechanism that is used to prioritize certain
traffic as it is forward through the switch. Traffic can be classified as High
or Normal priority and, when the switch is heavily loaded, it is the Normal
priority packets that are dropped first. You can select how traffic is
prioritized by using one of the three QoS modes, which are selected using
the QoS Mode drop-down list.
Note: Only one QoS mode can be active at one time. It is not possible,
for example, to prioritize traffic using the DSCP value and 802.1p
tag.
QoS Disabled
QoS is turned off and all packets have equal priority.
802.1p
Packets are prioritzed using the content of the VLAN-tag. The 802.1p field
is held within the VLAN-tag of a packet. The field is three bits long so can
hold eight values; 0 - 7 inclusive. When QoS Mode is set to 802.1p, the
802.1p Configuration table appears which allows a priority (normal or
high) to be set for each of the eight values.
You can use the Prioritize Traffic drop-down list to quickly set the values
in the 802.1p Configuration table. Select All Normal Priority to set all
values to normal priority or select All High Priority to set all values to high
priority. Use Custom if you want to set each value individually.
Note: Because end-stations, like PCs, are not usually VLAN aware, they
do not create VLAN-tagged frames. As a result, this method of
prioritization is not ideal when there are a lot of PCs connected to
the switch.
DSCP
Packets are prioritized using the DSCP (Differentiated Services Code
Point) value.