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 MGCP is a plain-text protocol used by call-control devices to manage IP telephony gateways.
MGCP (IETF RFC 2705) is a master/slave protocol that allows a call-control device (CUCM) to take
control of a specific port on a gateway. MGCP has the advantage of centralized gateway administration
and provides for scalable IP telephony solutions. CUCM controls the MGCP state of the configured
ports of the gateway. MGCP allows centralized administration of the dial plan and gives CUCM
232 Chapter 10: Configuring Voice Gateways
per-port control of all gateway interfaces. The gateway interfaces are traditional time-division
multiplexing (TDM) interfaces, including analog and digital interfaces, which can
interconnect PBXs, voice-mail systems, plain old telephone service (POTS) phones, and
other traditional device connections. MGCP is implemented with the use of a series of plaintext
commands (Session Description Protocol [SDP]) sent over User Datagram Protocol
(UDP) port 2427 from CUCM to the gateway.
It is also important to note that for an MGCP interaction to take place with CUCM, the
gateway and gateway interface must have CUCM support. Use the Feature Navigator tool
(http://www.cisco.com/go/fn) to make sure that the router or switch platform version of
Cisco IOS Catalyst Operating System (CatOS) software is compatible with the CUCM
version MGCP features.
Endpoint Identifiers
The MGCP call agent (CUCM) directs its commands to the gateway to manage an endpoint
or a group of endpoints on the device. Endpoint identifiers address individual endpoint
interfaces.
Endpoint identifiers consist of two logical parts. The first part is the local name of the
endpoint (hostname of fully qualified domain name). The two parts are separated by an
at symbol (@). If the local part represents a hierarchy, the subparts of the hierarchy are
separated by a slash (/). The local ID may be representative of a particular gateway/circuit
number, and the circuit number may in turn be representative of a circuit ID/channel
number.
Figure 10-1 shows a gateway with two endpoint interfaces. The show voice port summary
Cisco IOS command would display the hardware T1 or E1 voice WAN interface card
(VWIC) in slot 1, subslot 1, port 1 (1/1/1) and the Foreign Exchange Station (FXS) analog
interface in slot 2, subslot 1, port 1 (2/1/1). The T1 or E1 card provides the ability to trunk
24 (T1-CAS), 23 (T1-PRI), or 30 (E1) calls to the PSTN. The configuration of the T1 or
E1 interface is done primarily in CUCM for MGCP gateways. The first of CUCM’s two
configured endpoints (S1/SU1/DS1-1@gw1.domain.com) indicates that the T1 interface
was configured as an ISDN PRI using the DS1 (digital signal level 1) signaling
specification. The second interface is an analog access line number (AALN) that indicates
the interface is an FXS or Foreign Exchange Office (FXO) port. An FXS port generates dial
tone to the device plugged into the RJ-11 port. End-user devices such as fax machines and
analog phones are normally plugged into FXS ports. FXO RJ-11 ports accept dial tone and
are normally connected to the PSTN to make and receive phone calls. MGCP gateways do
not support caller ID services on FXO ports. If caller ID over analog FXO interfaces is a
requirement, H.323 gateways should be used. H.323 is covered in a later chapter.
Media Gateway Control Protocol Gateways 233
Figure 10-1 MGCP Endpoint Nomenclature
Both MGCP and Skinny Client Control Protocol (SCCP) are master/slave protocols; the
CUCM is the master server for both protocols. SCCP and MGCP devices cannot operate
without CUCM:
■ SCCP IP phones communicate directly with CUCM for all call setup signaling.
■ MGCP gateways communicate directly with CUCM for all call setup signaling.
Figure 10-2 displays a call that was received over an MGCP gateway to an SCCP IP phone.
The MGCP communication from the gateway is sent to the CUCM for call processing.
CUCM performs digit analysis on the received digits, and the lookup results in an SCCP
endpoint. CUCM sends information to the gateway, allowing the gateway endpoint to play
a ringback tone to the calling party. CUCM also sends SCCP signaling to the Cisco IP
Phone, indicating that there is an incoming call. When the destination Cisco IP Phone end
user answers the phone, CUCM coordinates the communication of the media path between
V
T1/E1 VWIC
1/1/1
MGCP
S1/SU1/DS1-1@gw1.domain.com
Port 1
Hostname
Hostname
Slot 1
Slot 2
Port 1
Subunit 1
Endpoint Type
(T1/E1 Trunk)
Subunit 1
Endpoint Type
(Analog Line)
AALN/S2/SU1/1@gw1.domain.com
PSTN/PBX
FXS VWIC
2/1/1
S1/SU1/DS1-1@gw1.domain.com
AALN/S2/SU1/1@gw1.domain.com
234 Chapter 10: Configuring Voice Gateways
the Cisco IP Phone and the MGCP gateway. The voice bearer traffic from the gateway to
the IP phone uses Real-Time Transfer Protocol (RTP). RTP uses an even port number in the
UDP port range of 16,384 through 32,767.
Figure 10-2 MGCP Call Flow
MGCP Gateway Support
MGCP support in CUCM includes a wide range of analog and digital interfaces that can be
used on several Cisco router and switch platforms. Tables 10-1, 10-2, and 10-3 list MGCPsupported
analog and digital features.
CUCM allows the Cisco IOS MGCP gateway to pull its MGCP-related configuration
from the Cisco TFTP server. This eliminates the need for manual MGCP gateway
configuration.
CUCM also supports Q.931 backhauling. Q.931 backhauling is supported on ISDN PRI,
BRI, and E1 using common channel signaling (CCS). The MGCP call agent (CUCM) takes
control of the ISDN D channel. The gateway sends CUCM all Q.931 call setup and
teardown traffic over TCP port 2428.