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Chapter 17 Quality of Service (QoS)
P-2612HNU-Fx User’s Guide
234
17.6.1 IEEE 802.1Q Tag
The IEEE 802.1Q standard defines an explicit VLAN tag in the MAC header to
identify the VLAN membership of a frame across bridges. A VLAN tag includes the
12-bit VLAN ID and 3-bit user priority. The VLAN ID associates a frame with a
specific VLAN and provides the information that devices need to process the frame
across the network.
IEEE 802.1p specifies the user priority field and defines up to eight separate traffic
types. The following table describes the traffic types defined in the IEEE 802.1d
standard (which incorporates the 802.1p).
17.6.2 IP Precedence
Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to
prioritize packets in a layer-3 network. IP precedence uses three bits of the eight-
bit ToS (Type of Service) field in the IP header. There are eight classes of services
(ranging from zero to seven) in IP precedence. Zero is the lowest priority level and
seven is the highest.
17.6.3 DiffServ
QoS is used to prioritize source-to-destination traffic flows. All packets in the flow
are given the same priority. You can use CoS (class of service) to give different
priorities to different packet types.
Table 67 IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Level 7 Typically used for network control traffic such as router configuration
messages.
Level 6 Typically used for voice traffic that is especially sensitive to jitter (jitter is the
variations in delay).
Level 5 Typically used for video that consumes high bandwidth and is sensitive to
jitter.
Level 4 Typically used for controlled load, latency-sensitive traffic such as SNA
(Systems Network Architecture) transactions.
Level 3 Typically used for “excellent effort” or better than best effort and would
include important business traffic that can tolerate some delay.
Level 2 This is for “spare bandwidth”.
Level 1 This is typically used for non-critical “background” traffic such as bulk
transfers that are allowed but that should not affect other applications and
users.
Level 0 Typically used for best-effort traffic.