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Overview of IEEE 820.1Q VLAN support
The switch supports IEEE 802.1Q standards-based VLANs. This standard describes
port-based VLANs as well as the methods to propagate VLAN memberships across
compliant devices using GARP VLAN Registration Protocol (GVRP). Each frame
contains information about the VLAN. This information is contained in a 4-byte tag
that is inserted into each frame. This tag contains information concerning the VLAN
that the device belongs to.
GVRP automates the configuration of VLAN information at the switch. When using
devices that support GVRP, VLANs will automatically be created on the switch
based on information being passed across the network from other GVRP-enabled
devices in frames referred to as GVRP PDUs. This further eases change and
movement as the administrator does not need to make any configuration changes
at the switch, the change will automatically be detected and the necessary VLAN
port membership changes made by the switch.
The switch provides configuration options that allow the use of devices that do not
support tagging or GVRP. With proper configuration, both “legacy” devices and
devices that support tagging or GVRP may be used on the same network.
These configuration options are described in the following sections.
Port VLAN ID (PVID)
The Port VLAN ID (PVID) specifies a VLAN ID for all untagged frames received on
the port. Only one PVID can be configured per port. This setting is used to
determine to which VLAN the untagged frames belong as they enter the switch. The
specific use of this value will be discussed later in this appendix.
GARP VLAN registration protocol (GVRP)
The switch provides a feature that allows the automatic propagation of VLAN
membership information across the network. This feature is facilitated by a new
protocol called GARP VLAN Registration Protocol (GVRP) that is defined as a part
of the IEEE 802.1Q standard. GVRP registration messages (PDUs) are sent across
the network and received by GVRP-enabled devices (switches, adapters, and so
on). This protocol allows devices to automatically join and leave VLANs. An
advantage of this is that if a user moves from one network connection point to
another, you would not have to manually reconfigure the switch ports to add the
new switch port to the VLANs that the user belongs to.
GVRP messages are sent across the network with a group address of
0x0180C2000021. The GVRP PDUs use the the same DYAP/SSAP as Spanning
Tree BPDUs. Older network analyzers often interpret these GVRP PDUs as
Spanning Tree BPDUs. The switch allows you to disable the GVRP function on a
switch basis or on an individual port basis.
Static versus dynamic VLANs
Two VLAN types, static and dynamic are associated with the switch. As the network
administrator, you can manually configure static VLANs. Dynamic VLANs are
created on the switch as a result of GVRP registration messages. Consequently, a
dynamic VLAN is automatically removed from the switch if it is no longer being
used by other devices in the network. You can convert a dynamic VLAN to a static
VLAN. Once this is done, the VLAN will remain configured on the switch until you
remove it.
For each static VLAN configured on the switch, you can define the mode of
participation for each port. There are three modes of participation:
Appendix E. Introduction to Virtual LANs (VLANs)
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