Before you configure VLANs, review the following key concepts.

Related Standards and Protocols

The following standards and protocols apply to the VLANs that you can configure:

• IEEE 802.1Q is a standard for VLANs. It aims to:

  • Define an architecture to logically partition bridged LANs and provide services to defined user groups, independent of physical location

  • Allow interoperability between multivendor equipment.

    IEEE 802.1Q defines the bridging rules for VLANs (ingress and egress rules, as described in detail in "Rules of VLAN Operation" later in this chapter). The standard also specifies a tag format that embeds explicit VLAN membership information within each frame in a 12-bit VLAN ID (VID), that provides 4094 possible VLANs. IEEE 802.1D, which now incorporates 802.1p, uses this same frame format but it takes advantage of an additional 3 bits to specify the priority levels used for Class of Service differentiation.

• Generic Attribute Registration Protocol (GARP) - This protocol is defined in IEEE 802.1p, which is a supplement to the IEEE 802.1D standard. GARP is a Layer 2 transport mechanism that allows switches and end systems to propagate information across the switching domain.

• GARP VLAN Registration Protocol (GVRP) - This protocol, which is defined in IEEE 802.1Q, defines dynamic registration of VLANs that use IEEE 802.1Q tagging (the VLAN ID). GVRP is supported for this release on Multilayer Switching Modules.

The system supports per-port tagging (that is, you can select IEEE 802.1Q tagging or no tagging on a per-port basis). Tagging and non-tagging ports can coexist in the same VLAN group.

• Non-tagging mode - The default tagging mode. Use this tagging mode for front-panel ports if the environment includes end stations that do not support 802.1Q VLANs. Non-tagged VLAN ports accept tagged frames; however, any traffic transmitted from an untagged port on a VLAN is untagged.

• 802.1Q tagging mode - With this form of tagging, VLAN frames are encapsulated and tagged as specified in the IEEE 802.1Q standard. In frame tagging mode, an explicit header that identifies to which VLAN the frame belongs is inserted into each frame of interswitch data. Frames in the same VLAN can be tagged or untagged. An untagged port in a VLAN cannot insert a tag, but it can recognize a tagged frame. Use this mode for VLANs in an IEEE 802.1Q environment.

You must evaluate tagging for each switching module's front-panel ports and backplane ports as well as the switch fabric module ports:

• For front-panel ports, you must use tagging when you have ports shared by different VLANs (VLANs that overlap on ports) and there is no other distinguishing characteristic. For port-based VLANs, tagging must be used to distinguish the shared ports (only one VLAN's shared front-panel ports can be untagged; in all other VLANs, the shared ports must be tagged). For VLANs on Multilayer Switching Modules, tagging is required to differentiate between shared ports of the same protocol type and overlapped IP Layer 3 VLANs in allClosed mode.

• For backplane ports and switch fabric module ports, you must use tagging when these ports are shared by multiple VLANs. (Only one VLAN's backplane ports can be untagged; in all other VLANs defined across the backplane, the backplane ports must be tagged.) If you tag the backplane port of a switching module for a VLAN, you must also tag the corresponding switch fabric module port in that VLAN.

  Devices (end stations, routers, switches, and so forth) that are connected to an explicitly tagged front-panel port must be capable of supporting 802.1Q tagging. If the front-panel port is untagged in the VLAN to which they belong, however, they do not have to support 802.1Q tagging.

Each VLAN is identified by its VLAN ID (VID). For VLANs that you create, the system keeps track of its used VLAN ID numbers to help you select the next available VLAN ID. Data frames sent by the system are tagged per IEEE 802.1Q (which contains the VID) if tagging is enabled on the transmit port for that VLAN. Tagged IEEE 802.1Q data frames that are received on the system are assigned to the VLAN that corresponds to both the VID contained in the tag and the protocol type.

Be aware of these additional guidelines:

• The default VLAN always uses the reserved VID of 1.

• Before assigning a VID, review the information in Table 49.

Table 49 Assigning ID Numbers to VLANs

VLAN ID Number	Description
VID 1	Reserved for the default VLAN assigned by IEEE and 3Com Corporation
VID 4095	Reserved
VID 2-4094	Numbers that you assign when you create VLANs

On Multilayer Switching Modules, if you rely on dynamic configuration to create a port-based VLAN based on GVRP updates, the VID is the unique IEEE 802.1Q VID.

If you define a router port IP interface, the system automatically creates a router port IP VLAN and assigns it the next available VID. See Chapter 16 for information on router port IP interfaces.

Terminology

Review the following terms:

• Default VLAN - The predefined port-based VLAN interface on all switch fabric module ports and the ports of each switching module that always uses

  • VID 1

  • Protocol type unspecified (for Multilayer Switching Modules)

  • The name Default

    The default VLAN also initially includes all of the bridge ports without any tagging, but you can modify the bridge ports and tag status of the default VLAN. See "The Default VLAN" for more detailed information.

• VLAN origin - Whether the VLAN was created in one of the following ways:

  • Statically - The VLAN display shows an origin of static if you define the VLAN.

  • Dynamically - The VLAN display shows an origin of GVRP if the system learned the VLAN dynamically through GVRP.

  • Router - The VLAN display shows an origin of router if you have defined a router port IP interface on a single bridge port. When you define a router port IP interface, you must place the system in allClosed mode. This setting removes any allOpen VLANs and re-creates the default VLAN. See Chapter 16 for more information on defining router port IP interfaces.

• Protocol suite - On Multilayer Switching Modules, the protocol family that is associated with a protocol-based or network-based VLAN. The protocol suite is unspecified for the default VLAN and all port-based VLANs.

• Layer 3 address - On Multilayer Switching Modules, the network or subnetwork address that is associated with a network-based IP VLAN.

• Port membership - The bridge ports that you assign to be part of the VLAN. If you have created trunks, you must specify the anchor port (lowest-numbered) port in the trunk when you define the VLAN. All bridge ports are initially part of the default VLAN on each module. See Chapter 12.

• VLAN name - The name that you assign to the VLAN. It can contain up to 32 ASCII characters. If the name includes spaces, enclose the name in quotation marks. The default VLAN always uses the name Default.

• Dynamic VLAN configuration - The method that enables dynamic VLAN configuration of port-based VLANs and dynamic updates of IEEE 802.1Q tagged port-based VLANs, using the GARP VLAN Registration Protocol (GVRP).

• Ingress and egress rules - Ingress rules that determine the VLAN to which an incoming frame belongs. If it cannot be assigned to any VLAN, it is assigned to the null VLAN, which contains no ports and has no associated address table in allClosed mode. Egress rules determine whether the frame is forwarded, flooded, or filtered, as well as the tag status of the transmitted frame. For more information on ingress and egress rules, see "Rules of VLAN Operation" later in this chapter.