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Cisco CCNA Voice Exam Cram: Configuring the Network to Support VoIP

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This chapter describes the typical boot process for Cisco IP phones and indicates the best practices related to configuring the network to support Voice over IP (VoIP).
This chapter is from the book

This chapter is from the book

To deploy Cisco Unified Communications Manager Express, network services need to be established and properly configured on all routers and switches for Cisco IP phones to function. This chapter describes the typical boot process for Cisco IP phones and indicates the best practices related to configuring the network to support Voice over IP (VoIP).

Understanding the Theory of Voice VLANs

A Cisco IP phone can act as a three-port switch. Just like a switch, the phone can support 802.1Q frames between itself and another switch, and more than one VLAN can be supported between the Cisco Unified IP phone and an access switch. Figure 7.1 illustrates how the Cisco IP phone acts as a three-port switch.

Figure 7.1

Figure 7.1 Cisco IP phones include a three-port switch.

The following are the three ports of the Cisco IP phone:

  • The external port that connects to a 10/100/1000 Ethernet switch
  • The external 10/100/1000 Ethernet port for PC connections
  • An internal 10/100/1000 Ethernet port for VoIP traffic

The benefits of this type of configuration include the following:

  • Cisco IP phones can be deployed on the network without IP address scalability problems. IP subnets usually have more than 50 percent—and often more than 80 percent—of their IP addresses allocated. A separate VLAN and its separate IP subnet to carry the voice traffic allow a large number of new devices, such as IP phones, to be introduced into the network without extensive modifications to the existing IP address scheme.
  • Voice and data VLANs allow the logical separation of data and voice traffic due to different characteristics. This separation allows you to handle each traffic type individually, applying different quality of service (QoS) policies to each VLAN for monitoring and managing them separately.

The following are IP addressing recommendations when adding Cisco IP phones to an existing data network:

  • Continue to use existing addressing for data devices (PCs, workstations, and so on).
  • Add Cisco IP phones and use DHCP to provision IP addresses and operating parameters.
  • Use new subnets for Cisco IP phones if they are available in the existing address space, or use private addressing such as the 10.0.0.0 network (see RFC 1918 for details) if subnets are not available in the existing address space.

With IP phones residing in a separate VLAN—a voice VLAN—it is easier for you to automate the process of deploying IP phones. The IP phone communicates with the switch, using the Cisco Discovery Protocol (CDP), to request the voice VLAN if present. The switch CDP response provides the phone with the appropriate 802.1Q VLAN ID, known as the voice VLAN ID (VVID). The PC traffic travels across the same connection without the 802.1Q tag inserted.

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