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40 Rapier Switch User Guide
Rapier Switch Software Release 2.2.1
C613-02013-00 Rev A
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.
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.
Table 5: Parameters displayed in the output of the SHOW VLAN command
Parameter Meaning