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Layer 2 Switching 85
Software Release 2.6.1
C613-02039-00 REV A
The Egress Rules
Once the Forwarding Process has determined which ports and transmission
queues to forward a frame from, the Egress Rules for each port determine
whether or not the outgoing frame is VLAN-tagged with its numerical VLAN
Identifier (VID). (See “Virtual Local Area Networks (VLANs)” on page 72).
When a port is added to a VLAN, it is configured to transmit either untagged
or VLAN tagged packets, using the command:
ADD VLAN={vlanname|1..4094} PORT={port-list|ALL}
[FRAME={TAGGED|UNTAGGED}]
This setting can be changed for a port which is already part of a VLAN, using
the command:
SET VLAN={vlanname|1..4094} PORT={port-list|ALL}
FRAME={UNTAGGED|TAGGED}
Quality of Service
The switch hardware has a number of Quality of Service (QOS) egress queues
that can be used to give priority to the transmission of some frames over other
frames on the basis of their user priority tagging. The user priority field in an
incoming frame (with value 0 to 7) determines which of the eight priority levels
the frame is allocated. When a frame is forwarded, it is sent to a QOS egress
queue on the port determined by the mapping of priority levels to QOS egress
queues. All frames in the first QOS queue are sent before any frames in the
second QOS egress queue, and so on, until frames in the last QOS egress queue,
which are only sent when there are no frames waiting to be sent in any of the
higher QOS egress queues.
The mapping between user priority and a QOS egress queue can be configured
using the command:
SET SWITCH QOS=P0,P1,P2,P3,P4,P5,P6,P7
The switch has four QOS egress queues. It has a default mapping of priority
levels to QOS egress queues as defined in IEEE Standard 802.1Q (Table 14).
Table 14: Default priority level to queue mapping for four QOS egress queues
Priority level QOS Egress Queue
01
10
20
31
42
52
63
73