Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 56)

Chapter 2 Campus Infrastructure Considerations

IP Addressing and Management

2-22

Cisco IP Telephony Network Design Guide

78-11103-03

CDP Enhancements

Cisco Discovery Protocol (CDP) is a device discovery protocol that runs on all

Cisco equipment. With CDP, each device sends periodic messages to a multicast

address and in turn listens to the periodic messages sent by other devices. This

allows devices on the network to discover one another and learn information such

as protocols used, protocol addresses, native VLAN of interconnected ports, and

so on. CDP is also used to send some Layer 2 and Layer 3 messages.

Cisco IP Phones use CDP to interact with the switch so that the switch knows that

an IP phone is connected to it. To provide this level of support, three new fields

have been added to CDP:

• Voice VLAN ID (VVID) for communicating the voice subnet to the IP phone

• Trigger field for soliciting a response from the connected device

• Power requirement field for getting the exact power requirement from the

phone

VVID Field

A VLAN (Layer 2) maps to a subnet (Layer 3) as a broadcast domain, such that a

VLAN is equivalent to a subnet. The VVID was introduced with release 5.5 of the

Catalyst software. This is the voice VLAN that the switch assigns to the IP phone

inside the CDP message. It allows the IP phone to get its VLAN ID automatically

when it is plugged into the switch if a VLAN is configured for the phone (see the

“Voice VLAN Configuration” section on page 2-24). If no VLAN is configured

for the IP phone, the phone resides in the native VLAN (data subnet) of the switch.

Trigger Field

The trigger field is used to force a response from the connected device. Under

normal circumstances, a device sends CDP update messages at a configured

interval (default is one minute). If an IP phone is connected between CDP

messages, it cannot receive its VVID. In this case, the IP phone issues a trigger in

the CDP message it sends to the switch, forcing the switch to respond with a

VVID.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 56