342-86400-498PS
Issue 1.2
April 2012
Page 157
Copyright
GE Multilin Inc. 2010-2012
APPENDIX B
VIRTUAL LOCAL AREA
NETWORK (VLAN)
In an Ethernet LAN, all broadcast plus unlearned-DA frames are sent to all
devices on the network. Multicast frames are broadcast unless their DA has
been entered as a “static” entry in the network switches‟ MAC address tables
(either manually or by a multicast routing protocol such as GMRP or IGMP).
Around 1998, the IEEE introduced a new “802.1Q” format for the Ethernet frames
to allow the “tagging” of frames with a “VLAN” (“Virtual LAN”) ID, and a
requirement that frames belonging to one VLAN would not enter any other VLAN.
This new format inserts a 4-
byte “tag” field after the frame‟s SA field; the first 2
bytes are the standard “type” field but with the value 8100/hex to flag this 4-byte
“tag”; the remaining 2 bytes comprise a 12-bit “VID” (VLAN ID) field, a 3-bit
priority field, and a 1-
bit “CFI” flag (zero for Ethernet switches).
If only the priority field is used, the VID field is 0, and the frame is called an
802.1p frame.
Note that the original maximum-frame-size of 1518 bytes now becomes 1522
bytes.
For networks comprising distinct groups of devices, each group can thus be
assigned to a different VLAN to avoid wasting bandwidth, and to provide some
degree of security.
Note
: VLAN is a logical grouping of Ethernet devices that reside in a common
broadcast domain.
Port-Based VLANs and Nested VLANs
A common way of implementing VLANs is to use switches which can “tag” the
ingressing frames from the local Ethernet devices (connected to the switches
“edge” ports) according to a user-configured, per-port, VLAN ID. On ETHER-
1000 unit, this parameter is referred to as D-PVLAN, while on the ETHER-100
