Network Router User Manual

SmartSwitch Router User Reference Manual 299
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:
ToS R ewrite
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:
• 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.)
Enable WRED on input or
output queue of specified
ports.
qos wred input|output [port <port list>|all-
ports][queue control|high|medium|low]
[exponential-weighting-constant <num>]
[min-queue-threshold
<num>][max-queue-
threshold
<num>][mark-prob-denominator num>]
MBZ
ToS
Precedence
7 6543210
Most Significant Bit Least Significant Bit