Allied Telesis AT-S26 - Manual (Page 97)

AT-S26 Version 2.0.1 User’s Guide
97
The ability to allow a port to forward traffic from many VLANs instead of
only one (as in port-based VLANs) allows the user to overcome the port-
based limitations mentioned above. A server can now be configured to
accept the traffic from many different VLANs, and then return data to the
various VLANs without mixing or leaking data into the wrong VLANs.
Now when propagating VLANs across the network you can use one port
per switch for connecting all VLANs on the switch to another 802.1Q-
based switch.
IEEE 802.1Q Standard
This flexibility comes from the ability to included a VLAN tag, in the form
of a VID, to an Ethernet frame, and the ability of NICs, switches, and
routers to act upon these VIDs (802.1Q).
The 802.3ac standard deals with the addition of 4 bytes to the original
802.3 frame. This means that while the minimum frame size is still 64
bytes, the maximum allowable frame size has been increased to 1522
bytes. These four bytes are inserted between the destination MAC
address field and Length/Type field and include the following
information.
Length/Type field (2 bytes): The Length/Type field of a tagged MAC
frame always uses the Type interpretation, and contains the 802.1Q Tag
Protocol Type: 0x81-00.
Tag Control Information field (2 bytes): The Tag Control Information field
is subdivided as follows:
a. A 3-bit User Priority field.
b. A Canonical Format Indicator (CFI) of 1 bit.
c. A 12-bit VLAN Identifier or VID.
The VID is the information that is used by the switch to forward the
frame to the appropriate VLANs. For further explanation of the function
and use of the other data fields, please consult the IEEE 802.1Q standard.
The 802.1Q standard deals with how this tagging information is used to
forward the traffic throughout the switch.
The handling of frames tagged with VIDs coming into a port is
straightforward. If the incoming frame’s VID tag matches one of the VIDs
of a VLAN that the port is a tagged member of, the frame will be
accepted and forwarded to the appropriate ports. If the frame’s VID does
not match any of the VLANs that the port is a member of, the frame will
be dropped.