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4. Chapter 4: Advanced Topics
This switch supports both Layer 2, which is based on physical device addresses, and
Layer 3 switching, which is based on IP network addresses. These functions, along with
other advanced features are described in this chapter.

4.1.Layer 2 Switching

When a frame enters a port, its destination MAC address is checked in the address
database to see which port leads to this destination. If the destination address belongs
to the incoming port, the frame is dropped or “filtered.” If the destination port is found on
another port, the frame is forwarded to that port and queued for output. But, if the
destination address is not found in the address database, the frame is sent to one or
more output ports based on the rules for handling tagged or untagged VLAN frames.
If the source MAC address of the frame was not found in the address database, it is
recorded along with the incoming port number where it entered the switch. This
information is then used to make later decisions for frame forwarding.
During switching, the switch performs multiple steps, including:
• VLAN Classification
• Learning
• Filtering
• Forwarding
• Aging
The following sections provide additional information about the tasks the switch
performs during unicast and multicast switching.

4.1.1.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