?– Simplifies the management of the LAN because, in effect, a large LAN is broken
down to smaller, usually more easily managed LANs (i.e., the logical LANs).
The IEEE 802.3ac standard allows for the Ethernet frame extension required to
accommodate a 4-Byte VLAN tag. The 4-Byte VLAN header comprises a 2-Byte VLAN
type (i.e., the inserted frame should be interpreted as a VLAN frame) and a 2-Byte
control field that, in turn, is made up of a 3-bit Priority field (called P bits), and a 12-bit
9 IEEE 802.3 frame defined in 1997. There is a slight variation between frames from the traditional and
DIX standards.
10 A PDU specifies a unit used to communicate between the same layers on different devices. It is
comprised of a header and a payload.
Ethernet: From LAN to the WAN 7
VLAN ID (VID). There are 4096 (212) unique VIDs.11 While this appears to be a fairly
large number and is sufficient in most LAN environments, it could present a bottleneck
(to scale) in larger and more complex enterprise environments and also when Ethernet
extends beyond the LAN, into Service Provider networks.12 The P bits are used to prioritize
the handling of incoming Ethernet frames.
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