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When deploying CUCM in multisite environments, centralized CUCM-based services are
accessed over the IP WAN. Affected services include the following:
• Signaling in CUCM multisite deployments with centralized call processing:
Remote Cisco IP Phones register with a centralized CUCM server. Remote MGCP
gateways are controlled by a centralized CUCM server that acts as an MGCP call
agent.
• Signaling in CUCM multisite deployments with distributed call processing:
In such environments, sites are connected via H.323 (non-gatekeeper-controlled,
gatekeeper-controlled, or H.225) or Session Initiation Protocol (SIP) trunks.
• Media exchange: RTP streams between endpoints located at different sites.
• Other services: These include Cisco IP Phone Extensible Markup Language (XML)
services and access to applications such as attendant console, CUCM Assistant, and
others.
Figure 1-3 shows a Unified Communications network in which the main site is connected
to a remote site through a centralized call-processing environment. The main site is also
connected to a remote cluster through an intercluster trunk (ICT) representing a distributed
call processing environment. The combination of both centralized and distributed call
processing represents a hybrid call-processing model in which small sites use the CUCM
resources of the main site, but large remote offices have their own CUCM cluster. On the
bottom left of Figure 1-3 is a SIP trunk, typically over a Metro Ethernet connection to an
Internet Telephony Service Provider (ITSP). The benefit of the SIP trunk is that the ITSP
provides the gateways to the PSTN instead of your providing gateways at the main site.
Dial Plan C h a l l e n g e s 9
Figure 1-3 Availability Challenges
Main Site Remote Cluster
An IP WAN outage in Figure 1-3 will cause an outage of call-processing services for the
remote site connected in a centralized fashion. The remote cluster will not suffer a callprocessing
outage, but the remote cluster will not be able to dial the main site over the IP
WAN during the outage. Mission-critical voice applications (voice mail, interactive voice
response [IVR], and so on) located at the main site will be unavailable to any of the other
sites during the WAN outage.
If the ITSP is using the same links that allow IP WAN connectivity, all calls to and from the
public switched telephone network (PSTN) will also be unavailable.
NOTE A deployment like the one shown in Figure 1-3 is considered badly designed
because of the lack of IP WAN and PSTN backup.