Port Priority and Transmit Queue Configuration
Fixed Switch Configuration Guide 17-15
Port Priority and Transmit Queue Configuration
The fixed switch devices allow you to assign mission-critical data to higher priority through the
device by delaying less critical traffic during periods of congestion. The higher priority traffic
through the device is serviced first before lower priority traffic. The Class of Service capability of
the device is implemented by a priority queueing mechanism. Class of Service is based on the
IEEE 802.1D (802.1p) standard specification, and allows you to define eight priorities (0 through 7)
and assign them to transmit queues for each port.
A priority 0 through 7 can be set on each port, with 0 being the lowest priority. A port receiving a
frame without priority information in its tag header is assigned a priority according to the default
priority setting on the port.
You can also map frame priorities to transmit queues, which allows you to configure which
transmit queue will be used for frames with specific priorities.
The arbitration methods used by transmit queues can be configured with the set port txq
command.
Setting Port Priority
Use the set port priority command to set the 802.1D (802.1p) Class-of-Service transmit priority
(0 through 7) on each port. A port receiving a frame without priority information in its tag header
is assigned a priority according to the priority setting on the port. For example, if the priority of a
port is set to 5, the frames received through that port without a priority indicated in their tag
header are classified as a priority 5 and transmitted according to that priority. .
A frame with priority information in its tag header is transmitted according to that priority.
Example
This example sets the 802.1p port priority to 4 on ports ge.1.1 through ge.1.3, then shows the
current settings.
System(su)->set port priority ge.1.1-3 4
System(su)->show port priority ge.1.1-3
ge.1.1 is set to 4
ge.1.2 is set to 4
ge.1.3 is set to 4
Mapping Port Priority to Transmit Queues
The set port priority-queue command enables you to map a transmit queue (0 to 5, with 0 being
the lowest priority queue) for each port priority of the selected port. You can apply the new
settings to one or more ports. Although ports have 8 queues, only queues 0 through 5 may be
configured. Queues 6 and 7 are reserved for management traffic.
Note: When CoS override is enabled using the set policy profile command, CoS-based
classification rules will take precedence over priority settings configured with the set port priority
command described in this section.
Note: Priority to transmit queue mapping on an individual port basis can only be configured on
Gigabit Ethernet ports (ge.x.x) and LAG ports (lag.0.x). On switches that provide Fast Ethernet
ports, when you use the set port priority-queue command to configure a Fast Ethernet port
(fe.x.x), the mapping values are applied globally to all Fast Ethernet ports on the system.
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