Information Technology Reference
In-Depth Information
CUCM Call Flows
Chapter 6, “Understanding the CME Dial-Plan,” discussed how Cisco Unified Communi-
cations Manager Express (CUCME) selects call-routing targets. The dial-plan in Cisco
Unified Communications Manager (CUCM) is more complex because it is a distributed
system that uses remote components, such as gateways to route calls. CUCM is intended
to scale to large-enterprise environments, and consequently has a greater capacity for call-
routing complexity and redundancy. This chapter introduces and discusses call signaling
and voice traffic flow in different scenarios, the components of the call-routing system,
the call-routing decision process, component configuration, redundancy, and restriction.
Call Flow in CUCM if DNS Is Used
Generally speaking, Domain Name System (DNS) is not recommended for use with Cisco
IP phones. If DNS is used, the IP phones must complete a DNS name resolution lookup to
learn the IP address of the CUCM server before any signaling can occur. At best, doing so
introduces delay; at worst, it allows the possibility of a misconfiguration or failure of the
DNS system that could cause the phones to stop working.
When the DNS lookup has completed successfully, the call flow consists of signaling (us-
ing either SCCP or SIP) between the phone and the CUCM, and the voice bearer streams
(using Real-Time Transport Protocol [RTP]) directly between the phones. Note that the
phones do not signal each other directly, nor does any voice traffic usually flow through
the CUCM. Figure 10-1 illustrates call flow when DNS is used by the phones.
Note:
The exception to the last statement is if the CUCM is hosting a voice conference;
in that case, the voice streams from all conference participants flow into the CUCM and
the combined streams (minus the listener's own stream) flow out of the CUCM back to the
participants.
Call Flow in CUCM if DNS Is Not Used
Eliminating IP phone reliance on DNS is recommended to eliminate unnecessary delay and
potential points of failure. If DNS is not used, the call flow is similar, except that the ini-
tial DNS lookup is eliminated, there remains only the signaling flow between the phones
and the CUCM, and the voice bearer streams directly between the phones. Figure 10-2
illustrates call flow without DNS in use by the phones.
The elimination of DNS reliance is simple to configure. The default installation of CUCM
lists the host name in the database field used by the phone configuration file to identify
the CUCM server(s) the phone should use for registration. To change the value in this
field, follow these steps:
1.
In CUCM Administration, navigate to
System
>
Server
.
