3Com 4007, Implementation Manual, Page: 202 / 684

202 C
HAPTER
10: C
LASS OF
S
ERVICE
(C
O
S)
Format of
Prioritized Packets
Priority level information can only be carried inside packets that are
formatted according to the IEEE 802.1Q standard; such packets carry an
extra 2 octets of data called a
tag . The priority level information occupies
3 bits of this tag and VLAN information occupies 12 bits.
The following definitions summarize the difference between tagged and
untagged packets and clarify two types of tagged packets:
■
Untagged packet — Does not include an IEEE 802.1Q tag.
■
Tagged packet — Includes an IEEE 802.1Q tag. There are two types:
■
Priority-tagged packet
— Carries priority level information but
no VLAN information.
■
VLAN-tagged packet — Carries priority level information and
VLAN information.
Queues and
Priority Levels
Compliance with the IEEE 802.1p standard means that a device must
recognize eight priority levels (0–7), however the number of queues in a
given device can vary. (Eight queues are not required.)
When there are fewer than eight device queues, a packet’s priority level
does not always indicate how it will be processed relative to other
packets. This is because more than one priority level will be assigned to at
least one of the queues. When multiple priority levels are assigned to the
same queue, all packets in that queue are processed in the same manner,
regardless of their priority level.
The characteristics of a given queue as well as overall product design
determine how the packets in that queue are processed relative to
packets in other queues. The device vendor identifies these
characteristics.