HP-UX VLAN Administrator's Guide, February 2007

Priority is a field in the VLAN tag portion of the frame header. ToS is part of the IP precedence
field in the IP header. Switches can use the 802.1p priority. Switches ignore ToS; routers can use
it.
In HP-UX, you can specify a 3-bit priority encoding (resulting in eight possible values) for each
VLAN configured on a NIC port. HP-UX VLANs conform to the IEEE 802.1p standard for priority.
The VLAN tag carries this value to all the switches on the route. Some switch vendors have
implemented a priority mechanism that acts on this 3-bit priority encoded in the VLAN tag (see
Figure 1-3 (page 17)) to provide a rudimentary Class of Service (CoS), or differentiated service.
For example, in the event of congestion, the switch might give a better service (such as lower
drop rate or higher scheduling priority) to frames carrying a certain 802.1p priority value in the
VLAN tag. For more information on priority policies on switches, see the switch manufacturers'
user documentation.
In HP-UX, a user can assign an 802.1p priority to a VLAN. This priority is subsequently encoded
in the VLAN tag of the frame’s Ethernet header. However, HP-UX does not enforce any priority
mechanisms in the end-station protocol stack, device drivers, or the NICs. HP-UX end stations
do not distinguish between frames with different 802.1p values in the VLAN tag.
The ToS octet is part of the IP precedence field in the IP header. Using well-known TCP/IP socket
options, applications can specify a specific ToS octet. But because switches are Layer 2 devices,
typically they do not monitor or act on the priority encoding of the ToS octet. Some switches do,
but this can have performance implications.
HP-UX VLANs support 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 examining 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.
User 802.1p Priority Settings
The priority field can be set by the user, destination address, input port, output port, access
priority, or VLAN. A user-specified priority enables priority information to be encoded in the
frame. The eight levels of IEEE 802.1p recommended user priorities and the corresponding ToS
values are listed in Table 1-2.
Table 1-2 ToS to 802.1p User Priority Mappings
Traffic TypeIEEE 802.1p User PriorityToS Value Range
Network management7 (highest)0xE0 to 0xFF
Voice60xC0 to 0xDF
Video50xA0 to 0xBF
Controlled load40x80 to 0x9F
Excellent effort30x60 to 0x7F
Best effort0 (routine traffic)0x40 to 0x5F
Undefined20x20 to 0x3F
Background1 (lowest)0x00 to 0x1F
Priority Override and ToS Override Mechanisms
Override mechanisms exist for both 802.1p priority and ToS such that the default IP precedence
levels in headers can be overwritten.
22 Introduction