Cisco ASA 5506-X Configuration Manual page 205

Chapter 8 Inspection for Voice and Video Protocols
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. The following figure illustrates how you can use NAT with MGCP.
Figure 8-1
Gateway is told
to send its media
to 209.165.200.231
(public address
of the IP Phone)
MGCP endpoints are physical or virtual sources and destinations for data. Media gateways contain
endpoints on which the call agent can create, modify and delete connections to establish and control
media sessions with other multimedia endpoints. Also, the call agent can instruct the endpoints to detect
certain events and generate signals. The endpoints automatically communicate changes in service state
to the call agent.
Gateways usually listen to UDP port 2427 to receive commands from the call agent.
•
The port on which the call agent receives commands from the gateway. Call agents usually listen to
•
UDP port 2727 to receive commands from the gateway.
MGCP inspection does not support the use of different IP addresses for MGCP signaling and RTP data.
Note
A common and recommended practice is to send RTP data from a resilient IP address, such as a loopback
or virtual IP address; however, the ASA requires the RTP data to come from the same address as MGCP
signaling.
Using NAT with MGCP
To PSTN
Cisco
PGW 2200
209.165.201.11
MGCP
209.165.200.231
GW
RTP to 209.165.201.1
from
H.323
M
M
209.165.201.10
209.165.201.1
SCCP
GW
209.165.200.231
Cisco ASA Series Firewall CLI Configuration Guide
MGCP Inspection
M
Cisco
CallManager
209.165.200.231
RTP to 10.0.0.76
from 209.165.200.231
IP IP IP
10.0.0.76
Branch offices
8-13