Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 65)

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Cisco IP Telephony Network Design Guide

78-11103-03

Chapter 2 Campus Infrastructure Considerations

Quality of Service

Figure 2-14 PC Is Not Trusted

The switch uses its queues (available on a per-port basis) to buffer incoming

frames before sending them to the switching engine. (It is important to remember

that input queuing comes into play only when there is congestion.) The switch

uses the CoS value(s) to put the frames in appropriate queues. The switch can also

employ mechanisms such as weighted random early detection (WRED) to make

intelligent drops within a queue (also known as congestion avoidance) and

weighted round-robin (WRR) to provide more bandwidth to some queues than to

others (also known as congestion management).

Example Scenario for the Catalyst 6000

Each port on the Catalyst 6000 family switches has one receive queue and two

transmit queues. On the receive side, all packets go into a regular queue. Tail drop

is used on this regular queue for congestion avoidance, but this mechanism comes

into play only if there is congestion on the receive side. This is unlikely in most

cases, because a frame coming in from a 10/100 Ethernet or Gigabit Ethernet port

onto a 32-Gbps bus will not experience congestion.

On the transmit side, CoS values 0, 1, 2, and 3 go into the low regular queue and

CoS values 4, 5, 6, and 7 go into the high regular queue. In addition, within each

queue WRED can be used to make intelligent drops based on the CoS value and

the percentage of buffers that are full. Finally, the high regular queue and low

regular queue are serviced based on the WRR configuration. These queues are

configurable; for example, they could be configured to be serviced in a 25 to 75

ratio.

Example: set port qos 2/1 trust-ext untrusted

CoS = 5

PC is untrusted

Phone ASIC

rewrites CoS = 0

CoS = 0 CoS = 7

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 65