HP-UX VLAN Administrator's Guide, February 2007

Figure 1-2 Using VLANs to Create Independent Broadcast Domains Across Switches
VLAN-Aware Switches
Network switches and end stations that know about VLANs are said to be VLAN aware. Network
switches and end stations that can interpret VLAN tags are said to be VLAN tag aware. To
implement a VLAN in your network, you must use at least VLAN-aware switches.
To understand how logical partitioning of a LAN infrastructure is done using VLAN, keep in
mind the fundamental operation of a traditional switched LAN. Following are the two main
rules to remember regarding the functioning of a regular LAN switch:
• When the switch receives a broadcast or multicast frame from a port, it floods (broadcasts)
the frame to all other ports on the switch.
• When the switch receives a unicast frame, it forwards it only to the port to which it is
addressed.
A VLAN-aware switch changes the previous two rules as follows:
• When the switch receives a broadcast or multicast frame from a port, it floods the frame to
only those ports that belong to the same VLAN as the frame.
• When a switch receives a unicast frame, it forwards it to the port to which it is addressed
only if the port belongs to the same VLAN as the frame.
• A unique number called the VLAN ID identifies each VLAN. It is a 12-bit field in the VLAN
tag. You can have a theoretical maximum of 4095 discrete VLANs in a network. Some
VLAN-tag-aware switches support a much smaller number of VLANs. The number of
VLANs supported must not be confused with the number of VLAN IDs that can be used.
Typically, no limitations exist on which VLAN IDs you can use to identify VLAN
groups—most switches support the entire range of the 12-bit value to be used.
To determine which VLAN a frame belongs to, note the following:
• A VLAN-aware switch can make the association based on attributes of the frame (such as
Ethernet and IP header content). Example attributes include destination MAC address, IP
address, TCP port, Network Layer protocol, and so on.
• Attributes such as the switch port on which the frame arrived can also be used. In this case,
the switch implicitly assigns a VLAN ID to all frames arriving on a given port.
• A frame can carry explicit VLAN information in a tag that is added to the Ethernet header
(explicit VLAN tagging). See Figure 1-3 (page 17) for the format of the VLAN tag.
Overview of VLANs 15