
21-34
Cisco Security Appliance Command Line Configuration Guide
OL-6721-01
Chapter 21 Applying Application Layer Protocol Inspection
Managing MGCP Inspection
• Configuring and Enabling MGCP Inspection, page 21-36
• Configuring MGCP Timeout Values, page 21-38
• Verifying and Monitoring MGCP Inspection, page 21-39
MGCP Inspection Overview
MGCP is a master/slave protocol used to control media gateways from external call control elements
called media gateway controllers or call agents. A media gateway is typically a network element that
provides conversion between the audio signals carried on telephone circuits and data packets carried over
the Internet or over other packet networks. Using NAT and PAT with MGCP lets you support a large
number of devices on an internal network with a limited set of external (global) addresses. Examples of
media gateways are:
• Trunking gateways, that interface between the telephone network and a Voice over IP network. Such
gateways typically manage a large number of digital circuits.
• Residential gateways, that provide a traditional analog (RJ11) interface to a Voice over IP network.
Examples of residential gateways include cable modem/cable set-top boxes, xDSL devices,
broad-band wireless devices.
• Business gateways, that provide a traditional digital PBX interface or an integrated soft PBX
interface to a Voice over IP network.
MGCP messages are transmitted over UDP. A response is sent back to the source address (IP address
and UDP port number) of the command, but the response may not arrive from the same address as the
command was sent to. This can happen when multiple call agents are being used in a failover
configuration and the call agent that received the command has passed control to a backup call agent,
which then sends the response. Figure 21-3 illustrates how NAT can be used with MGCP.
Figure 21-3 Using NAT with MGCP
119936
Cisco
CallManager
Gateway is told
to send its media
to 209.165.200.1
(public address
of the IP Phone)
M
IP
M
M
Cisco
PGW 2200
H.323
To PSTN
209.165.201.10
209.165.201.11
209.165.201.1
IP IP
Branch offices
RTP to 209.165.201.1
from 209.165.200.1
RTP to 10.0.0.76
from 209.165.200.1
10.0.0.76
209.165.200.1
MGCP
SCCP
GW
GW
209.165.200.1
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