Planning and Implementing VLANs with HP-UX

service (for example, lower drop rate or higher scheduling priority) to frames carrying a certain
802.1p priority value in the VLAN tag. For information on priority policies on switches, refer to
the switch manufacturer’s manuals.
HP-UX allows a user to assign an 802.1p priority to a VLAN. This priority is subsequently encoded
in the VLAN tag of the frame’s Ethernet header. However, at the time of this writing HP-UX
does not enforce any priority mechanisms in either the end station protocol stack, device drivers,
or the NICs. In other words, HP-UX end stations do not distinguish between frames with different
802.1p values in the VLAN tag.
IP ToS and 802.1p Conversion—End-to-End Class of Service
HP-UX allows you to map IPv4 Type of Service (ToS) octet to 802.1p priority. The ToS octet is a
field in the IP header. Using well-known TCP/IP socket options, applications can specify a desired
ToS octet. But since switches are Layer 2 devices, typically they do not look at or act on the
priority encoding of the ToS octet. Some switches do, but there may be performance implications.
HP-UX VLAN allows IP ToS octet to 802.1p priority conversion. Switches are more likely to
implement and enforce 802.1p priority with few or no performance implications because extracting
the priority from the VLAN tag is simpler than peeking into the IP header for the required
information. Using this mechanism, you can build a network with end-to-end class of service in
a LAN.
Typical Customer Configurations
The network shown in Figure 4 depicts a typical usage model. Note the following:
• Sets of workstations are grouped into VLANs, each possibly representing an IP subnet.
• HP-UX servers can be used to serve several VLANs at the same time, with a single point of
attachment to the LAN (for example, via a single NIC). This is accomplished by configuring
tagged VLANs on the NIC.
• You might want to use IP subnet-based VLANs on your backup server. This is advantageous
if you are backing up stations on more than one subnet.
• The servers must be VLAN aware, but the workstations need not be, as they typically tend
to belong to a single subnet. If you want to put a workstation NIC on several subnets, you
must make it tag aware.
• Using VLAN-aware servers and workstations, you will need only basic VLAN functionality
at the switches (for example, port-based VLAN and VLAN-tagging/-trunking capability).
VLANs on HP-UX 13