Understanding How VLANs Operate
Fixed Switch Configuration Guide 9-3
Preparing for VLAN Configuration
A little forethought and planning is essential to a successful VLAN implementation. Before
attempting to configure a single device for VLAN operation, consider the following:
• What is the purpose of my VLAN design? (For example: security or traffic broadcast
containment).
• How many VLANs will be required?
• What stations (end users, servers, etc.) will belong to them?
• What ports on the switch are connected to those stations?
• What ports will be configured as GARP VLAN Registration Protocol (GVRP) aware ports?
Determining how you want information to flow and how your network resources can be best used
to accomplish this will help you customize the tasks described in this document to suit your needs
and infrastructure.
Once your planning is complete, you would proceed through the steps described in “Configuring
VLANs” on page 9-8.
Understanding How VLANs Operate
802.1Q VLAN operation differs slightly from how a switched networking system operates. These
differences are due to the importance of keeping track of each frame and its VLAN association as
it passes from switch to switch, or from port to port within a switch.
VLAN-enabled switches act on how frames are classified into a particular VLAN. 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.
Learning Modes and Filtering Databases
Addressing information the switch learns about a VLAN is stored in the filtering database
assigned to that VLAN. This database contains source addresses, their source ports, and VLAN
IDs, and is referred to when a switch makes a decision as to where to forward a VLAN tagged
frame. Each filtering database is assigned a Filtering Database ID (FID). The FID a VLAN belongs
to can be displayed using the show vlan command.
A switch learns and uses VLAN addressing information by the following modes:
• Independent Virtual Local Area Network (VLAN) Learning (IVL): Each VLAN uses its own
filtering database. Transparent source address learning performed as a result of incoming
VLAN traffic is not made available to any other VLAN for forwarding purposes. This setting
is useful for handling devices (such as servers) with NICs that share a common MAC address.
One FID is assigned per VLAN. The FID value is the same as the VID it is assigned to. This is
the default mode on Enterasys switches.
For information about... Refer to page...
Learning Modes and Filtering Databases 9-3
VLAN Assignment and Forwarding 9-4
Example of a VLAN Switch in Operation 9-5
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