Allied Telesis C613-02013-00 manual VLAN Tagging

40		Rapier Switch User Guide
	Table 5: Parameters displayed in the output of the SHOW VLAN command
	Parameter	Meaning
	Protocol	The name of the protocol, which is determined from the
		format and identification number.
	Format	The encapsulation format specified by the module.
	Discrim	The discriminator specified by the module to identify which
		packets of the given format should be received.
	MAC Address	The Media Access Control source address for which the
		module wishes to receive packets. This is commonly known
		as the Ethernet address.

There are some disadvantages to using VLANs with untagged ports only:

■It is difficult to share network resources, such as servers and printers, across several VLANs. The routing functions in the switch must be configured to interconnect using untagged ports only.

■A VLAN that spans several switches requires a port on each switch for the interconnection of the various parts of the VLAN. If there are several VLANs in the switch that span more than one switch, then many ports are occupied with connecting the VLANs, and so are unavailable for other devices.

These disadvantages can be overcome with the versatility of VLAN tagging.

VLAN Tagging

VLAN tagging provides the advantages of more efficient and flexible use of switch ports and network resources, while maintaining the level of security given by port-based VLANs. With VLAN tagging, a port can belong to several VLANs. This means that network resources can be shared between different VLANs by configuring their ports to belong to more than one VLAN. Only one port is required on each switch to uplink (trunk) all VLAN traffic between two VLAN aware switches, as this port can be configured to belong to all VLANs on the switch.

Support for VLAN tagging is implemented in the switch according to IEEE Standard 802.1Q. Just as with untagged ports, tagged ports in a VLAN belong to the VLAN’s broadcast domain. A VLAN Identifier (VID) is defined for each VLAN, and this VID is used to switch traffic through a VLAN aware network so that frames are only transmitted on ports belonging to the VLAN. Other vendors’ VLAN aware devices on the network can be configured to accept traffic from one or more VLANs. A VLAN-aware server can be configured to accept traffic from many different VLANs, and then return data to each VLAN without mixing or leaking data into the wrong VLANs.

Every frame admitted by the switch has a VID associated with it, either because it already had a VLAN tag when it arrived, or because the VLAN for which the incoming port is untagged was associated with the frame when it was admitted. The switch only forwards the frame over those ports that belong to the VLAN specified by this VID. When the switch forwards a frame over a tagged port to another VLAN-aware device (for instance, another switch), it adds a VLAN tag (the same VID) to the frame. When the switch forwards the frame over an untagged port to a VLAN-unaware device, it transmits the frame as a VLAN-untagged frame, not including the VID in the frame.

Rapier Switch Software Release 2.2.1 C613-02013-00 Rev A