SRST w/ VG224 – Using MGCP for fun and profit

We recently had a client with a branch office that they wished to expand with 3x VG224 Analogue gateways. Unfortunately they were only running a 2811 ISR router which is limited to 36 ephones in SRST. Whilst VG224 ports consume no DLU’s in CUCM from a licensing perspective each VG224 ports is a seperate SCCP registration when failed over into SRST. To avoid forcing them to upgrade their existing SRST router we looked at solutions that could provide survivability whilst still being easy to manage maintain.

Option One : Configure the VG224 as H.323
This is a perfectly valid solution and will probably fit most peoples requirements. We chose not to do this for the following reasons:
  • Their existing 2811 voice gateway was already configured up with MGCP and was running perfectly fine. They have consistently deployed this type of gateway across the organisation. Using H.323 for the VG224’s would require converting the voice gateway to H.323 also and break standards.
  • It doesn’t provide the ability to monitor analogue ports from a central configuration database (Cisco Unified Communications Manager)
Option Two : Configure the VG224 as an MGCP gateway with fallback to H.323
This was the option we decided to go for as it provided centralised management, ability to failover without using SRST SCCP registrations and maintained configuration autonomy.
Key configuration points are outlined below:
1. Configure the gateway for MGCP as per usual:
ccm-manager fallback-mgcp
ccm-manager mgcp
no ccm-manager fax protocol cisco
ccm-manager music-on-hold
ccm-manager config server x.x.x.x y.y.y.y
ccm-manager config
2. Configure your VG224 gateway for MGCP in CUCM. You can confirm this is working when you have the CCM successfully configuring the gateway:
%CMAPP-6-CONFIG_DONE: Configuration by CCM is doneVG224#  sh run | i mgcp
ccm-manager fallback-mgcp
ccm-manager mgcp
mgcp
mgcp call-agent y.y.y.y 2427 service-type mgcp version 0.1
mgcp dtmf-relay voip codec all mode out-of-band
mgcp rtp unreachable timeout 1000 action notify
mgcp modem passthrough voip mode nse
mgcp package-capability rtp-package
mgcp package-capability sst-package
no mgcp package-capability res-package
no mgcp package-capability fxr-package
no mgcp timer receive-rtcp
mgcp sdp simple
mgcp fax t38 inhibit
mgcp rtp payload-type g726r16 static
mgcp profile default
You can can also confirm it is working with the following :
VG224#sh ccm-manager
MGCP Domain Name: VG224
Priority        Status                   Host
============================================================
Primary         Registered              y.y.y.y
First Backup    Backup Ready             x.x.x.x
Second Backup   NoneCurrent active Call Manager:    y.y.y.y
Backhaul/Redundant link port:   2428
Failover Interval:              30 seconds
Keepalive Interval:             15 seconds
Last keepalive sent:            16:07:55 WAST Jul 11 2011 (elapsed time: 00:00:12)
Last MGCP traffic time:         16:07:55 WAST Jul 11 2011 (elapsed time: 00:00:12)
Last failover time:             14:54:19 WAST Jul 11 2011 from (10.24.0.241)
Last switchback time:           14:54:49 WAST Jul 11 2011 from (10.24.0.240)
Switchback mode:                Graceful
MGCP Fallback mode:             Enabled/OFF
Last MGCP Fallback start time:  14:54:52 WAST Jul 11 2011
Last MGCP Fallback end time:    14:55:10 WAST Jul 11 2011
MGCP Download Tones:            Disabled
TFTP retry count to shut Ports: 2Configuration Auto-Download Information
=======================================
Current version-id: 1306736658-af125081-aa59-4451-b342-2c7091b18cc9
Last config-downloaded:00:00:00
Current state: Waiting for commands
Configuration Download statistics:
Download Attempted             : 31
Download Successful          : 31
Download Failed              : 0
Configuration Attempted        : 3
Configuration Successful     : 3
Configuration Failed(Parsing): 1
Configuration Failed(config) : 0
Last config download command: New Registration
FAX mode: disable
Configuration Error History:
3. Configure the VG224 for Fallback Mode as per usual:
application
global
service alternate Default
!
ccm-manager fallback-mgcp
!
4. Configure your VG224 ports in CUCM and on the VG224:
!
voice-port 2/0
cptone AU
timeouts ringing infinity
timing digit 50
timing inter-digit 50
caller-id enable
!
dial-peer voice 500 pots
service mgcpapp
destination-pattern 12345
port 2/0
!
5. Configure your dial-peers as required pointing them at the SRST gateway:
dial-peer voice 900 voip
destination-pattern .T
session target ipv4:1.1.1.1
6. Now on your SRST gateway you must configure appropriate H.323 dial-peers for routing the extension ranges to the VG224’s for when it is in mgcp-fallback:
dial-peer voice 123 voip
description Send to VG224
destination-pattern 2219X
session target ipv4:2.2.2.2
This solution will provide MGCP registration back to your CUCM for both your SRST Voice Gateway and VG224 when in service but allow for fallback to H.323 in a WAN failure scenarios. All IP handsets can still register SRST via SCCP to your gateway in WAN failure.
Caveats & Points of Interest

  • Supplementary Services are only available for the VG224 when they are registered via SCCP. Things like Audible Message Waiting Indicators, Call Transfer, Call Waiting, Distincitve Ring, Feature Access Codes and Speed Dial will not work when the ports are MGCP registered.
  • Because we are using this to provide fallback as well any changes to DN on the CUCM would still require updating of the destination-pattern under the pots dial-peer for that port.
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4 thoughts on “SRST w/ VG224 – Using MGCP for fun and profit

  1. Excellent Post. Really useful. This got us out of a ditch during implementation. Appreciate it. 10 Stars :)

    Cheers
    Rathish

  2. great article, very well written.
    this solution managed to allow us to fallback into SRST mode and make outbound calls. fantastic.
    however we cannot make internal calls to other IP phones or analogue devices on another VG202/ 204 from this device :(

    anyone have any ides?

  3. Is the other devices SCCP registered to the SRST router or in mgcp-fallback (h.323) ??

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