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The following sections provide additional information about the
tasks the switch performs during unicast and multicast switching.
Unicast Switching
This section describes VLAN classification, learning, filtering, and
forwarding for unicast switching.
•
VLAN Classification—When the switch receives a frame, it
classifies the frame in one of two ways:
- If the frame is untagged, the switch classifies the frame into
the default VLAN for the incoming port.
- If the frame is tagged, the switch uses the tagged VLAN ID to
identify the broadcast domain of the frame.
•
Learning—After VLAN classification, the switch checks the
<source MAC address, VLAN> pair in the address table to see
whether this pair is known.
- If unknown, the switch adds this pair to the address table.
- If known, the switch checks the pair for an incorrect Port ID.
If the PID associated with the pair in the address table is
different from the receiving port, the switch modifies the PID
in the address table.
•
Filtering—After learning the address, the switch checks:
- If the source or destination port is not in the forwarding state.
(For example, if it is in blocking state or has been disabled.)
- If the source or destination MAC address is to be filtered.
- If the source PID is the same as the destination PID.
If any of these conditions are met, the switch drops the received
frame. Otherwise, it continues with the forwarding process as
described below.
•
Forwarding—During the forwarding process, the switch checks
whether the <destination MAC address, VLAN> pair is unknown.
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