Understanding the network.pdf - Back to Home

with a specific VLAN. A ULP-based VLAN is an example of using
implicit tagging for VLAN identification. Although implicit frame
tagging is easy to implement on a standalone switch to establish VLAN
port membership, there is no easy way to ensure that an implicitly
tagged frame will be forwarded correctly when it needs to leave the
switch for delivery. To ensure proper delivery of implicitly tagged
frame to the correct VLAN when it reaches a remote switch, some type
of centralized configuration data must be used to determine a frame's
VLAN assignment. For this reason, VLAN mechanisms utilize implicit
tagging operate in conjunction with a VLAN configuration server.
Switches installed on the network exchange configuration and VLAN
mapping information. Although using a central server makes
configuration easier, a server failure can have a severe impact on the
network's operation.
With explicit packet tagging, an extension to the Layer 2 frame a tag
is added to a packet as it enters the switch. The tags are used by the
switches in the LAN to indicate VLAN membership. However, use of
tagging adds to the length of the packet, so tagged packets will be
seen as deformed packets by switches that do not support tagging
and will be discarded.
With a marketplace full of incompatible solutions, a community effort
was taken up in cooperation with the IEEE to develop a standard for
"Virtual Bridged Local Area Networks," or standards-based VLANs.
The effort began in 1996 between the major LAN switch
manufacturers. Two solutions were considered. One was the
exchange of VLAN information between switches, a process similar to
the exchanging of dynamic routing tables between routers. The other,
based on Cisco Systems' ISL solution, was the use of packet tagging.
The tagging solution was selected by the IEEE to be the basis of the
IEEE 802.1q standard. This method, incidentally, is also the more
efficient of the two, because the exchange of VLAN information
between switches would only increase the amount of traffic on the
network.
IEEE 802.1q Standards-Based VLANs
The IEEE 802.1q standard provides an architectural model for the
configuration, distribution, and operation of VLANs. It is a three-layer
model consisting of a configuration layer, a distribution/resolution
layer, and a mapping layer.