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Example 1, Single Switch Operation
802.1Q VLAN User’s Guide 4-3
The switch will now classify each frame received as belonging to either
the Red or Blue VLANs. Traffic from one VLAN will not be forwarded to
the members of the other VLAN, and all frames transmitted by the switch
will be normal, untagged Ethernet frames.
4.1.2 Frame Handling
This section describes the operations of the switch when two frames are
received. The first frame is a broadcast sent by station R1.
1. Station R1 transmits the broadcast frame. The switch receives this
frame on Port 1. As the frame is received, the switch classifies it. The
frame is untagged, so the switch classifies it as belonging to the VLAN
that Port 1 is assigned to, the Red VLAN.
2. At the same time, the switch adds the source MAC address of the
frame and the VLAN associated with port 1 to its Source Address
Table in FID 2. In this fashion it learns that station R1 is located out
Port 1.
3. Once the frame is classified, its destination MAC address is examined.
The switch discovers that the frame is a broadcast, and treats it as it
would any other unknown destination MAC address. The switch
forwards the frame out all ports in the Red VLAN’s Forwarding List
except for the one that received the frame. In this case, the frame is
sent to Ports 2 and 3.
The second frame is a unicast, where station R2 responds to station R1’s
broadcast.
4. Station R2, having received and recognized the broadcast from R1,
transmits a unicast frame as a response. The switch receives this frame
on Port 2. The switch classifies this new untagged frame as belonging
to the Red VLAN.
5. The switch adds the source MAC address and VLAN for station R2 to
its Source Address Table in FID 2, and checks the Source Address
Table for the destination MAC address given in the frame. The switch
finds the MAC address and VLAN in this table, and recognizes that
the MAC address and VLAN match for R1 is located out Port 1.