Planning and Implementing VLANs with HP-UX

Which VLAN Does a Frame Belong To?
The previous section notes that a frame can belong to a VLAN. The next question is—how is this
association made?
• A VLAN-aware switch can make the association based on various 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 2 for the format of the VLAN tag.
Figure 2 IEEE 802.1Q VLAN Tag in Ethernet Frame
Destination
Address
Source
Address
Type/Len
Data
4 Bytes
2 Bytes 2 Bytes (Tag Control Information)
Frame
Check
Tag
Protocol
ID
0x8100
User
Priority
(3 Bits)
VLAN ID
(12 Bits)
Canonical
Format
Indicator
(1 Bit)
802.1Q
VLAN Tag
How Does a VLAN-aware Switch Work?
You can configure VLAN-aware switches to add ports to a VLAN group or groups. These switches
maintain two simple, related tables:
– a list of ports that belong to each VLAN enabled on the switch
– the set of VLANs enabled on each port
Several varieties of VLAN-aware switches are available:
• The most basic of these 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 , which is explained in following sections. 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
What is VLAN? 9