Planning and Implementing VLANs with HP-UX

Benefits of VLANs
The key benefits of using VLANs include the following:
• Bandwidth preservation: A well-designed VLAN helps restrict broadcast and multicast
traffic to only those stations listening to and responding to the traffic related to that VLAN.
The network and computing resources of nonparticipating stations are unaffected, thus
improving performance.
• Manageability: Moves, additions, and changes to network topology do not require physical
changes to network topology. User mobility is much simpler because of the dynamic nature
of VLANs.
Physically dispersed work groups can be logically connected within the same broadcast
domain to appear as if they are on the same physical LAN. A single physical link can
simultaneously serve several IP subnets when subnet-based VLANs are configured on that
link. End stations using VLANs can offer rudimentary Class of Service (CoS) locally by
prioritizing traffic for certain activities.
• Enhanced security: You can construct different security domains to provide various levels
of security in the network, because the network design is more flexible than that of traditional
LANs. Since frames are passed to a destination port only if the port belongs to the same
VLAN as the frame, VLANs help enforce traffic isolation, thereby providing an added level
of security in the network.
VLAN-Aware Switches Are the Key
To implement a VLAN in your network, you must use VLAN-aware switches. This section
describes how VLAN-aware switches are different from traditional switches.
To understand how logical partitioning of a LAN infrastructure is done using VLAN, it is helpful
to remember the fundamental operation of a traditional switched LAN. Without going into the
details of switch design, the two rules to remember regarding the functioning of a regular LAN
switch are:
1. When the switch receives a broadcast or multicast frame from a port, it floods (broadcasts)
the frame to all other ports on the switch.
2. 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 above two rules as follows:
1. 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.
2. 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.
3. A unique number called the VLAN ID identifies each VLAN
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. It is a 12-bit field in the VLAN
tag. You can have a theoretical maximum of 4095 discrete VLANs in your network.
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2. Most switches allow you to assign a name to each VLAN.
3. Some 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.
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