3Com Switch 8800 Advanced Software V5 Configuration Guide

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HARDWARE-BASED CONGESTION
MANAGEMENT CONFIGURATION
When configuring traffic classification and traffic shaping, go to these sections for
information you are interested in:
■ “Congestion Management Overview” on page 847
■ “Configuring SP Queues” on page 848
■ “Configuring Group-based WRR Queues” on page 849
Congestion
Management
Overview
As to a network device, congestion will occur on the interface where the arrival
rate of packets is faster than the sending rate. If there is no enough buffer capacity
to store those packets and then a part of them will be lost, which may cause the
packet retransmission from the device because of timeout, and lead to a vicious
circle.
The key to congestion management is how to define a dispatching policy for
resources to decide the forwarding order of packets when congestion occurs. This
chapter describes hardware-based congestion management configuration.
SP Queuing Strict priority (SP) queues include:
■ Basic SP queues: a basic SP queue contains multiple queues, with each queue
corresponding to a different priority. These queues are scheduled in the
descending order of priority.
■ Multi-mode SP queues: the queue scheduling modes are extended from that of
basic SP queues.
Multi-mode SP queues operate in one of the following three modes:
■ SP mode 0: in this mode, multi-mode SP queues are all basic SP queues and are
scheduled in the descending order of priority.
■ SP mode 1: in this mode, when the remaining external memory space is
sufficient, SP queue scheduling algorithm is adopted; when the remaining
external memory space is 0, the scheduling algorithm can preferentially
forward the packets stored in the internal memory of the chip even if packets
with higher priority are waiting to be scheduled in the external memory.
■ SP mode 2: in this mode, packets stored in the internal memory of the chip are
forwarded preferentially; if no packets are stored in the internal memory of the
chip, all the packets are scheduled using the SP queue scheduling algorithm.
The disadvantage of SP mode 2 is that the bus bandwidth of the external
memory is decreased.
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Currently, only SP mode 0 (that is, SP queue scheduling algorithm) is available to
Switch 8800s .