Cabletron Systems SSIM-R8-02 Especificações descarregar pdf (Página 339)

Enterasys Xpedition User Reference Manual 319

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 queues of specific ports, enter the following command in

Configure mode:

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:

• 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 XP 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 [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

1 2 ... 334 335 336 337 338 339 340 341 342 343 344 ... 393 394

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