VLAN Switch Operation
VLAN Operation and Network Applications 13-9
13.7 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 13-2 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 Filtering Database Identifier (FDB ID) 2,
VLAN C and D with FDB ID 3, and VLAN E with FDB ID 4. Port 6 has been classified to serve
as a VLAN trunk connection (will only transmit and receive tagged frames). 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 FDB ID 1. Although untagged frames are not usually
present on a VLAN trunk connection, any untagged frames received would need to be classified if
the port has not been configured to drop all untagged frames.
Figure 13-2 View from Inside the Switch
NOTE: The example in Figure13-2 shows ports sharing the same FDB ID, which is
supported by the switch. However, this feature cannot be configured using Local
Management at this time, but it can be configured using an SNMP management
application.
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