Content

| 7.6 |  | Network Design Issues for Voice |
| 7.6.6 |  | IP addressing and management |

Each IP phone requires an IP address, along with associated information such as subnet mask, default gateway, and so on. Essentially, this means that an organization's need for IP addresses doubles as IP phones are assigned to all current PC users.

This information can be configured statically on the IP phone, or can be provided by the Dynamic Host Configuration Protocol (DHCP).

Assigning IP Addresses Using Same Subnet as Data Devices
A straightforward solution to provide IP addresses to the IP phones may be using the same subnet as data devices. However, many sites have IP subnets with more than 50% of subnet addresses already allocated. This would require addition of address ranges or reallocation of the existing ranges.

Creating a Separate IP Subnet for IP Phones
IP phones can be placed on a separate IP subnet. The new subnet could be in a registered address space or in a private address space, such as network 10.0.0.0. Using this scheme, the PC would be on a subnet reserved for data devices and the phone would be on a subnet reserved for voice. Configuration on the IP phone can be minimized by having the phone learn as much information dynamically as possible. Therefore, when the IP phone powers up it should get its voice subnet automatically, then send a DHCP request on that subnet for an IP address.

The automated mechanism by which the IP phone gets its voice subnet is provided through enhancements to the Cisco Discovery Protocol (CDP).

CDP Enhancements
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. 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.

Power Requirement Field
When the switch provides inline power to an IP phone, it has no way of knowing how much power the phone needs (this varies by model). Initially, the switch allocates 10W, then adjusts the delivered power according to the requirements sent by the IP phone in the CDP message.