10-21
Port Status and Basic Configuration
QoS Pass-Through Mode
For example, suppose you have configured port 10 to assign a priority level
of 1 (low):
■ An untagged packet coming into the switch on port 10 and leaving the
switch through any other port configured as a tagged VLAN member
would leave the switch as a tagged packet with a priority level of 1.
■ A tagged packet with an 802.1p priority setting (0 - 7) coming into the
switch on port 10 and leaving the switch through any other port configured
as a tagged VLAN member would keep its original priority setting (regard-
less of the port-based priority setting on port 10).
Note For a packet to carry a given 802.1p priority level from end-to-end in a network,
the VLAN for the packet must be configured as tagged on all switch-to-switch
links. Otherwise the tag is removed and the 802.1p priority is lost as the packet
moves from one switch to the next.
Operating Rules for Port-Based Priority
These rules apply to the operation of port-based priority on the switch.
■ In the switch’s default configuration, port-based priority is configured as
“0” (zero) for inbound traffic on all ports.
■ On a given port, when port-based priority is configured as 0 - 7, an inbound,
untagged packet adopts the specified priority and is sent to the corre-
sponding outbound queue on the outbound port. (See table 10-3, “Mapping
Priority Settings to Device Queues”, on page 10-20.) If the outbound port
is a tagged member of the applicable VLAN, then the packet carries a tag
with that priority setting to the next downstream device.
■ On a given port, an inbound, tagged packet received on the port with a
preset priority of 0 - 7 in its tag keeps that priority and is assigned an
outbound queue on the basis of that priority (regardless of the port-based
priority configured on the port). (Refer to table 10-3, “Mapping Priority
Settings to Device Queues” on page 10-20.)
■ If a packet leaves the switch through an outbound port configured as an
untagged member of the packet’s VLAN, then the packet leaves the switch
without a VLAN tag and thus without an 802.1p priority setting.
■ Trunked ports do not allow non-default (1 - 7) port-based priority settings.
If you configure a non-default port-based priority value on a port and then
add the port to a port trunk, then the port-based priority for that port is
returned to the default “0”.
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