19216811.liveChapter 11 Quality Of Service
P-660R-F1 Series User’s Guide
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11.5
QoS Technical Reference
This section provides some technical background information about the topics covered in this
chapter.
11.5.1
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).
Table 59
IEEE 802.1p Priority Level and Traffic Type
11.5.2
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.
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LABEL
DESCRIPTION
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.