HP-UX VLAN Administrator's Guide, February 2007

You can configure VLAN-aware switches to add ports to a VLAN group or groups. VLAN-aware
switches maintain two simple, related tables: a list of ports that belong to each VLAN enabled
on the switch, and the set of VLANs enabled on each port.
Several varieties of VLAN-aware switches are available:
• The most basic VLAN-aware switches support port-based VLANs. In a port-based VLAN,
the switch port on which the frame arrived determines the VLAN membership of the frame.
These switches cannot support more than one VLAN per switch port unless they support
VLAN tagging (see “ VLAN Tagging”). A simple port-based VLAN that supports VLAN
tagging is all you need to implement a VLAN in an HP-UX environment.
• More sophisticated switch offerings enable users to configure VLAN membership rules
based on frame content, such as MAC address, TCP/UDP port, IP address, and so on. Doing
this can affect switch performance.
• VLAN-aware Layer 3 switches (or routing switches) perform the function of Layer 3 (e.g.,
IP routing) in addition to VLAN classification.
With regard to other network devices, note the following:
• You can configure an end station to belong to more than one VLAN.
• Shared bandwidth devices, such as hubs, cannot be VLAN aware, though they can be
included in a VLAN environment. If a hub is used in a VLAN environment, all nodes on
that hub must belong to the same VLAN or set of VLANs, thereby restricting the benefits
of VLANs.
• A common misconception is that because multiple IP subnets can share a single switched
infrastructure using VLANs, switching can replace routing in the network. Remember that
VLAN is strictly a Data Link Layer (Layer 2) technology. You must use routers for
communication between IP subnets, even in a VLAN.
VLAN Tagging
As mentioned previously, you can implement VLAN functionality via explicit frame tagging by
end stations or switches. Recall that 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. VLAN-tag-aware end stations add VLAN tags to standard
Ethernet frames–a process called explicit tagging. In explicit tagging, the end station or switch
determines the VLAN membership of a frame and inserts a VLAN tag (see Figure 1-3) in the
frame header, so that downstream link partners can examine just the tag to determine the VLAN
membership. The VLAN tag identifies which VLAN a data frame belongs to and enables traffic
from more than one VLAN to use the same switch or LAN card port (see Figure 1-4).
16 Introduction