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Recently there were few discussions in CSC on Implementing PSTN gateways in CUCM with queries related to configuration and troubleshooting. To help the users, I have decided to write this blog and provide configuration and troubleshooting steps to make the gateway implementation easy in the Service Provider (SP) network. Also I have been trying to capture all the day to day issues we face in the SP network and address the common issues in implementing the PSTN gateways in CUCM.

I have created this blog based on my work experience in implementing PSTN Gateways in the Service Providers (SP) network and I will cover the Basics, Configuration and Troubleshooting part.

Pl share your inputs and the issues you have faced in your network in the comments section and also let us know if you are looking for any further information or solution related to this topic.

To help all types of readers I will start from Basics and then I will cover the Network design, Configuration and Troubleshooting part.

PSTN Gateways in CUCM

To place external calls, Cisco Unified Communications Network (CUCM) deployment needs a connection the Public Switched Telephone Network (PSTN). PSTN connections are provided through Gateways, which connect traditional time-division-multiplexing (TDM) telephony interface (digital T1/E1 or analog FXO port) and VOIP domains.

Gateways can be integrated in CUCM by using different protocols such as Media Gateway Control Protocol (MGCP), H.323 or Session Initiation Protocol (SIP) for signaling on VOIP call legs.

Gateway Protocol functions for CUCM Integration

Three main signaling protocols - MGCP, H.323 and SIP provides different feature support. There are Pros and Cons in each protocol.

MGCP: Centralized dial plan configuration and Centralized gateway configuration hence it will be easy to implement in a large SP network.

H.323: Dial plan configured directly on the gateway. More specific call routing and it support third-party integration on SP network.

SIP: Dial plan configured directly on the gateway. It supports third-party telephony integration and end devices.

MGCP Gateway Considerations

A constant logical IP session (connection) must be present between the Cisco IOS MGCP gateway and CUCM. If the connection between CUCM and the MGCP gateway isunavailable, the following can happen:

  • If the gateway was configured for failover (by using the ccm-manager fallback-mgcp command), the MGCP gateway can fail over to local call control mode. All active calls are dropped (no call survivability/call preservation), and a complete local dial plan must be present. Fallback-MGCP functionality is normally enabled on branch gateway routers configured with SRST in centralized call-processing architectures.Fallback-MGCP can be configured on headquarters routers as well, but this functionality is normally not present because the MGCP gateway’s CUCM group defined in the gateway’s device pool can include up to three CUCM servers to register the MGCP gateway to. Local MGCP registration failovers will not result in disconnected calls at the headquarters because of the graceful switchover feature that enables call survivability. No softkey features will be available during the duration of the survived calls, and new inbound and/or outbound calls will not work until the gateway reregisters with a CUCM server.

  • If no failover configuration is present, all calls are dropped, and the PRI interface goes down.

Gateway Integration with CUCM

In this blog I will be discussing the following Gateway integration.

1) Integrating MGCP gateways with CUCM

2) Integrating H.323 gateways with CUCM

3) Integrating SIP gateways with CUCM

A gateway is a device that can translate between different types of signaling and media. One type of gateway is a voice gateway. Voice gateways are used anytime that a Cisco IP Phone communicates with TDM interface. For example, PSTN, traditional PBX, Analog phone, fax and so on.

1) Integrating MGCP gateways with CUCM

  • MGCP is a plain-text protocol that call-control devices use to manage the IP telephony gateways.
  • MGCP is a client/server protocol that allows the call agent (CA) to take control of a specific gateway endpoint (port).
  • MGCP has the advantage of centralized gateway administration in the CUCM. CUCM controls the state of each port on the gateway endpoint .
  • MGCP gateway can be controlled on a per endpoint (TDM port) level but H.323 and SIP cannot.

Note: MGCP gateway must be supported in CUCM. Use the Cisco software advisor Tool to make sure that the platform and version of Cisco IOS software or Cisco Catalyst Operating system is compatible with MGCP for CUCM.

MGCP Gateway Support:

MGCP support in CUCM includes a wide range of analog and digital interfaces that can be used ton several Cisco router and switch platforms. FXO, FXS, T1-CAS, T1-PRI, E1-CAS, E1-PRI but some voice ports are not supported by MGCP say, E&M voice ports. CUCM pushes the Cisco IOS MGCP gateway configuration from the Cisco TFTP server to the gateway when automatic configuration is configured.

CUCM also supports Q.931 backhaul. Q.931 backhaul is only supported on ISDN voice ports. MGCP backhaul allows CUCM to process the Q.931 messaging from the ISDN circuit. The gateway router encapsulates the Q.931 signaling from the gateway over TCP port 2727.

MGCP Call flow:

MGCP communication is used between the CUCM and the MGCP Gateway.

MGCP call flow.bmp

MGCP Configuration Server:

Configuration Server feature allows all MGCP configurations to be administered through the GUI in CUCM. Cisco IOS commands are downloaded automatically from the TFTP server to the MGCP gateway. Figure shows the MGCP configuration sever communication.

mgcp server configuration.bmp

Q.931 Backhaul:

Q.931 backhaul is a reliable TCP transport layer TCP/IP connection between CUCM and Cisco MGCP gateway. Q.931 backhaul is an encapsulation of the Q.931 signaling in the D channel of the ISDN TDM interface. Q.931 backhaul carries the raw Q.931 signaling to CUCM to be processed natively. The Gateway is still responsible for the termination of the Layer 2 Q.921 Link Access Protocol D-Channel (LAPD) signaling, but all ISDN layer 3 call setup/call tear down signaling (Q.931) is sent to CUCM for processing.

1.1) MGCP Gateway Configuration: CUCM

Add an MGCP Gateway to CUCM

Step1: In CUCM Administration, Choose Device > Gateway.

Step2: Click the Add New button.

Step3: Choose the Appropriate MGCP gateway by gateway or router name

[ Gateway Type * - Cisco IAD2400/Cisco 1751/Cisco 1760/Cisco 1861/Cisco 269x/Cisco 26xx/Cisco 2801/Cisco 2811/Cisco 2821/Cisco 2821/Cisco 2851/Cisco 2901/Cisco 2811/Cisco 2821/Cisco 2851/Cisco 2901/Cisco 2911/Cisco 2921/Cisco 2951/Cisco 362x/Cisco 364x/Cisco 366x/Cisco 3725/Cisco 3745 etc)

Say in this case, Gateway Type: Cisco 2811

Step4: Click Next.

Step5: Choose MGCP from the protocol drop-down menu and click Next

MGCP Gateway Configuration

Sample MGCP gateway configuration displayed on the CUCM Page:

Select the type of gateway you would like to add:

Gateway Type Cisco 2811

Protocol * - options are MGCP / SCCP

In this case for MGCP gateway, select MGCP as the Protocol

For the detailed MGCP Gateway Configuration on CUCM with the step-by-step procedure, refer the document which I have created seperately to associate with this blog,

Note: To make this blog easy to read and understand, I'vecovered all the detailed procedures in seperate document for better clarity. All the key configuration and verification procedures are covered in this blog. I've captured all the below given information from my LAB CUCM and MGCP gateway.

Detailed MGCP Gateway Configuration

Sample MGCP gateway configuration display from the CUCM Administration Page:

Gateway Details

Product: Cisco 2811

Protocol: MGCP

Domain Name * : HQ-1

Description: HQ-1 headquarters MGCP gateway

Cisco Unified Communications Manager Group*: Default

Configured slots, VICs and Endpoints

Module in Slot 0

Module in Slot 1

(To add modules refer the next section “Endpoint module configuration”

Product specific configuration layout

Global ISDN Switch type: 4ESS

Switchback Timing* Graceful

Type of DTMF Relay* Current GW config

Modem Passthrough* Enable

MGCP Endpoint configuration

Sample MGCP Endpoint configuration, from the CUCM administration page

Device Information

Product: Cisco MGCP E1 Port

Gateway: HQ-1

Device Protocol: Digital Access PRI

End Port Name* S0/SU1/DS1-0@HQ-1

Description S0/SU1/DS1-0@HQ-1

Device Pool* Default

Location Hub_None

Enable : PSTN Access

Interface information

PRI Protocol Type* PRI EURO

QSIG Variant* No changes

Protocol side* User

Channel selection order* Bottom up

PCM Type* A-law

1.2) MGCP Gateway Configuration: Cisco IOS Configuration

Only the two following commands are required to configure an MGCP gateway. Assume the TFTP server IP address is

# ccm-manager config

# ccm-manager config server

Configuring MGCP Gateway Registration

Controller E1 0/3/0

framing crc4

linecode hdb3

pri-group timeslots 1-31 service mgcp


Interface serial0/3/0:15

isdn switch-type primary-4ess

isdn incoming-voice voice

isdn bind-13 ccm-manager


ccm-manager mgcp

ccm-manager music-on-hold



mgcp call-agent 2427 service-type mgcp version 0.1

mgcp rtp unreachable timeout 1000 action notify

mgcp modem passthrough voice mode nse

mgcp package-capability rtp-package

mgcp package-capability sst-package

mgcp package-capability pre-package

no mgcp package-capability res-package

no mgcp package-capability fxr-package

no mgcp timer receive-rtcp

mgcp sdp simple

mgcp rtp payload-type g726r16 static


1.3) MGCP Gateway: Registration Verification

Verifying MGCP Gateway Registration

Router# show ccm-manager

MGCP domain name : Router

Priority Status Host

Primary Registered

First Backup None

Second Backup None

Current active Call Manager:

Backhaul/Redundant link port: 2428

Failover Interval: 30 seconds

Keepalive Interval: 15 seconds

Last keepalive sent: 00:45:31 (elapsed time: 00:00:04)

Last MGCP traffic time: 00:45:31 (elapsed time: 00:00:04)

Last failover time: None

Switchback mode: Graceful

MGCP fallback mode: Not selected

Last MGCP Fallback start time: 00:00:00

Last MGCP Fallback end time: 00:00:00

PRI Backhaul Link info

Link Protocol: TCP

Remote Port Number: 2428

Remote IP Address:

Current Link State: OPEN


Packets recvd: 32

Recv failures: 0

Packets xmitted: 32

Xmit failures: 0

PRI Ports being backhauled: Slot 1, port 0


Configuration Auto-Download information

No configurations downloaded

Current State: Automatic Configuration Download feature is disabled

Configuration Error


FAX mode: cisco

1.4) Verifying MGCP Endpoint Registration

Verifying Endpoint Registration

Router# show mgcp endpoints

Interface T1 1/0



S1/ds1-0/1@AV-2620-4 1/0:23 none UP

S1/ds1-0/2@AV-2620-4 1/0:23 none UP

S1/ds1-0/3@AV-2620-4 1/0:23 none UP

S1/ds1-0/4@AV-2620-4 1/0:23 none UP

S1/ds1-0/5@AV-2620-4 1/0:23 none UP

S1/ds1-0/6@AV-2620-4 1/0:23 none UP

S1/ds1-0/7@AV-2620-4 1/0:23 none UP

S1/ds1-0/8@AV-2620-4 1/0:23 none UP

S1/ds1-0/9@AV-2620-4 1/0:23 none UP

S1/ds1-0/10@AV-2620-4 1/0:23 none UP

S1/ds1-0/11@AV-2620-4 1/0:23 none UP

S1/ds1-0/12@AV-2620-4 1/0:23 none UP

S1/ds1-0/13@AV-2620-4 1/0:23 none UP

S1/ds1-0/14@AV-2620-4 1/0:23 none UP

S1/ds1-0/15@AV-2620-4 1/0:23 none UP

S1/ds1-0/16@AV-2620-4 1/0:23 none UP

S1/ds1-0/17@AV-2620-4 1/0:23 none UP

S1/ds1-0/18@AV-2620-4 1/0:23 none UP

S1/ds1-0/19@AV-2620-4 1/0:23 none UP

S1/ds1-0/20@AV-2620-4 1/0:23 none UP

S1/ds1-0/21@AV-2620-4 1/0:23 none UP

S1/ds1-0/22@AV-2620-4 1/0:23 none UP

S1/ds1-0/23@AV-2620-4 1/0:23 none UP

1.5) Fractional T1/E1 Configuration on MGCP Gateway

In some situations, not all time slots of a T1 or E1 connection will be used. This type of PRI is called a fractional T1 or E1, depending on what part of the world you’re in. You can specify the number of usable B channels in Cisco Unified Communications Manager by setting the Cisco CallManager service parameter Change B-Channel Maintenance Status for the individual B channels on the ISDN PRI. Five PRI endpoints can be configured to have B channels in maintenance status. The B-Channel Maintenance Status setting has no effect on the XML configuration file that is received through the MGCP configuration server. The PRI group on the Cisco IOS MGCP gateway will always allocate the maximum number of B channels that are available for a specific controller type, but CUCM will never route a phone call to one of the configured B channels because the CUCM configuration indicates that certain B channels are out of service. To configure fractional T1 or E1 on the Cisco IOS gateway, a manual gateway configuration would need to be performed that only specified the number of active time slots in the respective ds0-group (T1/E1 CAS) or pri-group (T1/E1 PRI) Cisco IOS commands.

The configuration server functionality can be disabled in the router, and the preexisting MGCP configuration received from the TFTP server will be maintained. The fractional PRI group on the corresponding T1 or E1 controller is then configured. Anytime that Cisco IOS or CUCM versions are upgraded, I recommend testing the ccm-manager config server commands in a lab environment to see whether the new versions enabled any

new MGCP commands (MGCP package capabilities and so on) that might be required for the new version of Cisco IOS and CUCM to communicate properly. It’s always best to use the automatic configuration commands unless there’s a requirement for a fractional T1 or E1. Fractional circuits are not very common anymore in North America and other technically advanced countries because of dropping circuit costs.

Note:- The maximum number of PRI group B channels that can be supported in the gateway router depends on the number of installed voice interfaces and digital signal processors (DSP). T1 and E1 interfaces share DSP resources with hardware media resources configured in CUCM (transcoding, conferencing, and media termination point).

1.6) Fractional T1/E1 Configuration on CUCM

To put specific time slots of an MGCP T1 or E1 PRI into the maintenance state, you need to retrieve the MGCP endpoint ID, example : S0/SU1/DS1-0@HQ-1 and select the Enable status Poll check box in the interface information configuration section on the MGCP endpoint configuration page.

From the CUCM Administration, choose System > Service Parameter and then choose the Cisco Call Manager Service from the Service drop-down menu. Click the Advanced button at the top of the configuration page to view hidden advanced configuration options on the MGCP gateway that are required to change the B-Channel Maintenance status parameter.

This parameter allows CUCM to change the individual B-Channel maintenance state for the time slots on the T1/E1 interface.

The following is an example of a fractional T1 interface that has four active channels with top-down channel selection order:

0000 1111 1111 1111 1111 1111

1.7) Verifying T1/E1 Controller Status

Rourter # show controller e1

  • E1 status should be UP.
  • E1 should be Channelized E1 – balanced , 120 ohms
  • No Alarms Detected.

Verifying T1/E1 Controller status

HQ-1 ## show controller e1

  • E1 status should be UP.
  • E1 should be Channelized E1 – balanced , 120 ohms
  • No Alarms Detected.

E1 7/0 is up.

Applique type is Channelized E1 - balanced

Description: E1 from Idea

No alarms detected.

alarm-trigger is not set

Version info of slot 7: HW: 1032, PLD Rev: 7

Framer Version: 0x9

Manufacture Cookie Info:

EEPROM Type 0x0001, EEPROM Version 0x01, Board ID 0x03,

Board Hardware Version 4.8, Item Number 73-3996-05,

Board Revision A1, Serial Number JAE1125M7VQ,

PLD/ISP Version <unset>, Manufacture Date 24-Jun-2007.

Framing is NO-CRC4, Line Code is HDB3, Clock Source is Line.

Data in current interval (770 seconds elapsed):

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Data in Interval 1:

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Data in Interval 2:

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Data in Interval 3:

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Data in Interval 4:

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Data in Interval 5:

0 Line Code Violations, 0 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins

0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

1.8) Verifying T1/E1 Interface status

An easy way to check the operation of an MGCP controlled T1/E1 ISDN PRI interface is by using the

# show isdn-status command and checking the Layer1 and Layer2 status as demonstrated here.

Verifying T1/E1 Interface status

Router# show isdn status

  • Layer1 should be ACTIVE
  • Layer 3 should be ACTIVE

HQ-1 # show isdn-status

Global ISDN Switchtype = primary-net5

ISDN Serial 0/1/0:15 interface

ds1 0, interface ISDN Switchtype = primary-net5

Layer 1 Status:


Layer 2 Status:


Layer 3 Status:

0 Active Layer 3 Call(s)

Active dsl 0 CCBs = 0

The Free Channel Mask: 0xFFFF7FFF

Number of L2 Discards = 746, L2 Session ID = 4

Total Allocated ISDN CCBs = 0

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