Cabletron Systems 9032578-05 manual Weighted Random Early Detection Wred, ToS Rewrite

Chapter 21: QoS Configuration Guide

Weighted Random Early Detection (WRED)

Random Early Detection (WRED) alleviates traffic congestion issues by selectively dropping packets before the queue becomes completely flooded. WRED parameters allow you to set conditions and limits for dropping packets in the queue.

To enable WRED on input or output queues of specific ports, enter the following command in Configure mode:

Enable WRED on input or output queue of specified ports.

qos wred inputoutput [port <port list>all- ports][queue controlhighmediumlow] [exponential-weighting-constant <num>] [min-queue-threshold <num>][max-queue- threshold <num>][mark-prob-denominator num>]

ToS Rewrite

In the Internet, IP packets that use different paths are subject to delays, as there is little inherent knowledge of how to optimize the paths for different packets from different applications or users. The IP protocol actually provides a facility, which has been part of the IP specification since the protocol’s inception, for an application or upper-layer protocol to specify how a packet should be handled. This facility is called the Type of Service (ToS) octet.

The ToS octet part of the IP specification, however, has not been widely employed in the past. The IETF is looking into using the ToS octet to help resolve IP quality problems. Some newer routing protocols, like OSPF and IS-IS, are designed to be able to examine the ToS octet and calculate routes based on the type of service.

The ToS octet in the IP datagram header consists of three fields:

7	6	5	4	3	2	1	0
Precedence				ToS			MBZ
Most Significant Bit					Least Significant Bit

•The three-bit Precedence field is used to indicate the priority of the datagram.

•The four-bit ToS field is used to indicate trade-offs between throughput, delay, reliability, and cost.

•The one-bit “must be zero” (MBZ) field is not currently used. (In the SSR configuration, there is no restriction on this bit and it is included as part of the ToS field.)

SmartSwitch Router User Reference Manual 299