Information Technology Reference
In-Depth Information
CDP
CDP is a Cisco-proprietary Layer 2 protocol that provides network mapping information
todirectlyconnectedCiscodevices.(YoulearnedaboutCDPinyourCCNAstudies,so
we do not detail it here.) Cisco IP Phones generate CDP messages and use CDP to learn
the voice VLAN ID from the Cisco switch to which they are connected. The IP Phone then
tags the voice frames it is transmitting with that VLAN ID in the 802.1Q/P frame header.
DHCP
DHCP is a widely used IP standard that can provide the following information to IP Phones:
IP Address
■
Subnet Mask
■
Default Gateway
■
DNS Server(s)
■
TFTP Server(s)
■
Although it is possible to statically configure IP Phones with all that information, it would
be time-consuming and error-prone. DHCP is faster, easier, more scalable, and a widely ac-
cepted practice. DHCP can be provided by an existing DHCP server (because most de-
ployments already have one), a local router, or even by CUCM itself, although this is not
generally recommended, particularly for larger deployments. Later sections review the
configuration of DHCP services in CUCM and router IOS.
PoE
PoE is a standards-based feature that delivers DC power supply over Ethernet cabling. IP
Phones can use this feature, and doing so means less cabling to clutter the desk, no power
supplies to buy for the phones, and potential cost savings. PoE is generally assumed to be
provided by the switch that the phones connect to, but it may also be provided by a pow-
ered patch panel or inline power injector.
TFTP
TFTP is a critical service for IP Phones. The phones use TFTP to download their config
files, firmware, and other data. Without TFTP, the phones simply do not function prop-
erly. When you make a configuration change to a device, CUCM creates or modifies a
config file for the device and uploads it to the TFTP server(s). TFTP services must there-
fore be provided by one (or more in large deployments) of the CUCM servers in the clus-
ter; a generic TFTP server will not have the integrated capability that a CUCM TFTP
server does and will not correctly fulfill the role.
DNS
DNS provides hostname-to-IP address resolution. DNS services are not critical to IP
Phones. (In fact, in most deployments, it is recommended to eliminate DNS reliance from
the IP Phones [see Chapter 10, “Understanding CUCM Dial-Plan Elements and Interac-
tions”].) But in some circumstances, it is desirable. A DNS server must be external to the
CUCM cluster; DNS is not a service that CUCM can offer.
