HP TopTools for Hubs and Switches - Manual (Page 169)

Managing Switches
Configuring Switch Features
13-13
the “normal” outbound port queues. However, with a CoS policy operating in
your network, you can determine the outbound priority queue to which a
packet is sent.
Policy Options for Extending Traffic Priority Control to Outbound
Packets Traveling to Downstream Devices.
If an outbound packet is in an 802.1Q tagged VLAN environment, that is, if the
packet is assigned to a tagged VLAN on the outbound port, then the packet
carries an 802.1p priority setting that was configured in the switch. This
priority setting can range from 0 to 7, and can be used by downstream devices
that have up to eight queues. While packets within a switch move only at high
or normal priority, they still can carry the 802.1p priority that can be used by
downstream devices that have more than two priority levels. Also, if the
packet enters the switch with an 802.1p priority setting, CoS can override this
setting if configured appropriately.
N o t e If you are not using multiple tagged VLANs in your network, you can still use
the tagged VLAN feature available in HP Procurve switches by configuring the
default VLAN as a tagged VLAN.
Priority Settings for Outbound Packets. You can configure a CoS
priority of 0 through 7 for an outbound packet. When the packet is sent to a
port, the CoS priority determines which outbound queue the packet uses.
Table 13-3. CoS Priority
If a packet is not in an 802.1Q tagged VLAN environment, the above settings
control only to which outbound queue the packet goes, and no 802.1p priority
is added to the packet. However, if the packet is in an 802.1Q tagged VLAN
environment, then the above setting is also added to the packet as an 802.1p
priority that can be used by downstream devices and applications, as shown
in the Priority Mapping Table. For each priority policy setting this table shows:
■ The corresponding outbound port queue a packet will use when exiting
from any switch covered by the policy
■ The corresponding 802.1p priority setting the packet will carry when it
leaves the switch
QoS Setting Outbound Port Queue Operation
0*-3 normal Packets in this queue leave the port after the
high-priority queue is emptied.
4-7 high priority Packets in this queue leave the port first.
* Note that in compliance with the 802.1p standard, “0” is ranked as “normal” and is a higher
priority than “1” and “2”.