While voice signaling and media transport protocols, such as MGCP, H.323, SIP, SCCP, and RTP, are essential, other generic infrastructure protocols, such as DNS, TFTP, NTP, and Power over Ethernet (PoE), are also required in order for a telephony network to operate properly.
DNS, TFTP, NTP, and PoE are discussed in this chapter.
Cisco CallManager, Cisco IP phones, and other devices in a network can use DNS to resolve names to IP addresses. If the DNS server is down or inaccessible, or if the DNS entry is incorrect, failures can occur. For example, it is possible for Cisco IP phones to use DNS during registration, but registration will fail if they are unable to resolve the name of the CallManager.