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Chapter 2:
VLAN Operation
2-4 802.1Q VLAN User’s Guide
Figure 2-1 depicts the inside of a switch with six ports, numbered one
through six. The switch has been configured to associate VLAN A and B
with FID 2, VLAN C and D with FID 3, and VLAN E with FID 4. Port 6
has been classified as a 1Q Trunk Port. This classification establishes that
all VLANs are members of the Port VLAN List for Port 6 and the frames
transmitted for all VLANs will contain a tag header. Also the PVID for
Port 6 is set to the default VLAN with its corresponding relationship to
FID 1. Although untagged frames are not usually present on a 1Q Trunk
Port, any untagged frames received would need to be classified if the port
has not been configured to drop all untagged frames.
Figure 2-1 Inside the Switch
2.4.1 Receiving Frames from VLAN Ports
When a switch is placed in 802.1Q Operational Mode, every frame
received by the switch must belong, or be assigned, to a VLAN.
Untagged Frames
The switch receives a frame from Port 1 and examines the frame. The
switch notices that this frame does not currently have a VLAN tag. The
switch recognizes that Port 1 is a member of VLAN A and classifies the
frame as such. In this fashion, all untagged frames entering a VLAN
switch assume membership in a VLAN.
NOTE
A VLAN ID is always assigned to a port. By default, it is the
Default VLAN (VLAN ID = 1).
Port 4
D
FID 3
2599-02
Port 3
C
FID 3
Port 6
Port 2
B
FID 2
Port 1
A
FID 2
Default
FID 1
Port 5
E
FID 4