User's Manual

Configuration Guide of XGS-5240-Series
29-1
Chapter 29 Egress QoS Configuration
29.1 Introduction to Egress QoS
In traditional IP networks, all packets are treated in the same way. All network equipments
treat them by the first-in-first-out policy and try best effort to send them to the destination.
However, it does not guarantee the performance like reliability and transmission delay.
Network develops so fast that new demand has been raised for the quality of service on IP
network with the continual emergence of new applications. For example, delay-sensitive
services like VoIP and video put higher demands on packet transmission delay and users
cannot accept too long transmission delay (by contrast, E-mail and FTP services are not
sensitive to the time delay). In order to support services with different service requirement like
voice, video and data service, the network is required to be able to distinguish between
different communications and provide appropriate service. The traditional best-effort IP
network cannot identify and distinguish various kinds of communications while this ability is the
very premise of providing differentiated services for different communications. Therefore, the
best-effort service mode of traditional network cannot meet the demand of applications. The
emergence of QoS techniques is committed to solve this problem.
Egress PolicyMap is the QoS policy in egress which performs QoS control of packets in
the egress direction and provides better service for specified network communication with
kinds of techniques. Egress PolicyMap includes class-map and policy-map, of which
class-map is used for selecting packets to operate and policy-map is used for specifying the
operation to use. Not all equipments support Egress QoS currently.
29.1.1 Egress QOS Terms
Egress QoS: Achieving QoS on egress of port.
Inner_vid: VLAN ID brought by the TAG near the header of network layer when double TAGs
exist.
Outer_vid: VLAN ID brought by the TAG near the header of network link layer when double
TAGs exist. The TAG is considered to be outer tag by default when only one TAG exists.
Outer_tpid: Protocol type of the network link layer header indicating the type of outer tag.
29.1.2 Basic Egress QoS Model