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Chapter 46 QoS Configuration
If you care to use your bandwidth and your network resources efficiently, you must pay attention to QoS
configuration.
46.1 QoS Overview
46.1.1 QoS Concept
In general, the switch works in best-effort served mode in which the switch treats all flows equally and tries its
best to deliver all flows. Thus if congestion occurs all flows have the same chance to be discarded. However
in a real network different flows have different significances, and the QoS function of the switch can provide
different services to different flows based on their own significances, in which the important flows will receive
a better service.
As to classify the importance of flows, there are two main ways on the current network:
5. The tag in the 802.1Q frame header has two bytes and 3 bits are used to present the priority
of the packet. There are 8 priorities, among which 0 means the lowest priority and 7 means
the highest priority.
6. The DSCP field in IP header of the IP packet uses the bottom 6 bits in the TOS domain of
the IP header.
In real network application the edge switch distributes different priorities to different flows based on their
significance and then different services will be provided to different flows based on their priorities, which is the
way to realize the terminal-to-terminal QoS.
Additionally, you can also configure a switch in a network, enabling the switch to process those packets with
specific attributes (according to the MAC layer or the L3 information of packets) specially. This kind of behaviors
are called as the one-leap behaviors.
The QoS function of the switch optimizes the usage of limited network bandwidth so that the entire performance
of the network is greatly improved.
46.1.2 Terminal-To-Terminal QoS Model
The service model describes a group of terminal-to-terminal QoS abilities, that is, the abilities for a network to
transmit specific network communication services from one terminal to another terminal. The QoS software
supports two kinds of service models: Best-Effort service and Differentiated service.