Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 64)

Chapter 2 Campus Infrastructure Considerations

Quality of Service

2-30

Cisco IP Telephony Network Design Guide

78-11103-03

Traffic Classification at Layer 2

Cisco IP Phones can mark voice packets as high priority using CoS as well as ToS.

By default, the phone sends 802.1Q tagged packets with the CoS and ToS set to a

value of 5. Figure 2-13 shows packets from the IP phone being sent as tagged

frames with the 802.1p fields set to 5 and frames from the PC being sent untagged.

Figure 2-13 Frame Tagging with PVID and VVID

Because most PCs do not have an 802.1Q capable network interface card (NIC),

they send the packets untagged. This means that the frames do not have a 802.1p

field. Also, unless the applications running on the PC send packets with a specific

CoS value, this field is zero. A special case is where the TCP/IP stack in the PC

has been modified to send all packets with a ToS value other than zero. Typically

this does not happen, and the ToS value is zero.

Even if the PC is sending tagged frames with a specific CoS value,

Cisco IP Phones can zero out this value before sending the frames to the switch.

This is the default behavior and is illustrated in Figure 2-14. Frames coming from

the phone have a CoS of 5 and frames coming from the PC have a CoS of 0. When

the switch receives these frames, it can take into account these values for further

processing based on its capabilities.

Untagged 802.3

Tagged 802.1q

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 64