Enterasys Networks 802.1Q - Manual (Page 19)

VLAN Switch Operation
VLAN Operation 2-3
2.4 VLAN SWITCH OPERATION
IEEE 802.1Q VLAN switches act on the classification of frames into VLANs. Sometimes, VLAN
classification is based on tags in the headers of data frames. These VLAN tags are added to data
frames by the switch as the frames are transmitted out certain ports, and are later used to make
forwarding decisions by the switch and other VLAN aware switches. In the absence of a VLAN
tag header, the classification of a frame into a particular VLAN depends upon the configuration of
the switch port that received the frame.
The operation of an 802.1Q VLAN switch is best understood from a point of view of the switch
itself. To illustrate this concept, the examples that follow view the switch operations from inside
the switch.
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
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