March 9, 2009


Do you remember the first IPT call you made in your lab or at work??

I’ve got a three year old daughter. You’ll be amazed how much one can learn from these little ones.
Apparently, she’s been watching me configure my “mini-voice-lab” and making calls between HQ and Branch 1.
One of my HQ phones is a 7940 and the BR1 phone is an IP Blue soft-phone running on my laptop.

Now, either she knew what the number to dial, or she actually knew the redial button…….but all I heard was “Daddy!!!!! It is WORKING!!!!!!!”

Apparently, she made a call between the HQ and BR1 IP Phones and she could hear her voice coming out of my laptop……EUREKA!!!!!!!
I couldn’t get her away from the phone after that. I think the most fascinating thing for her was that she could hear herself through my laptop speaker. LOL!!  She kept saying “I can hear my voice”.

Now that feeling took me back to 2002…..the first time EVER I configured VoIP!!!!
The truth was I had no clue what I was doing back then.
I was a zealous intern working at a company called United Telesys (now called Emperion West Africa).

I was working with Sly, a senior engineer tasked with setting up voip for two branches of a bank (I think it was either Intercontinental Bank or Diamond Bank). One branch was located in Trans-Amadi, while the other branch was located in Olu-Obasanjo, both in Port Harcourt, Nigeria.

I remember we simulated the bank’s WLAN and had two 1750’s on each end of the wireless connection.
One 1750 had an FXO, while the other had an FXS………..I can remember the feeling too well……..I was so overwhelmed when the phones rang.

I’m sure every IPT/UC Engineer has had this 1st Call feeling before……..please don’t forget it cos it’s a memory to cherish 😉


March 1, 2009

In the course of my CCIE studies, I’ve found the “Gatekeeper Address Resolution on ARQ” chart extremely useful.
Having a good understanding of the call-flow displayed the chart is key to understanding the debug gatekeeper main 10 and how the gatekeeper logically looks through configured parameters to decide how a call will be routed.

The chart (shown below) should be available in the CCIE Voice Lab exam. It can be found on Page 10-40 (pg 350 figure 10-14) in the CCM 4.1 SRND, which we’ve been told would be available in the lab.





Now back to the Hop-off Tech Prefix.
While there are many ways to route calls on a gatekeeper, we may need to use the Hop-off Tech-prefix configuration scenario in the lab if we’re specifically asked to do that, or if we’re instructed not to use the following:

 – Tech prefix
 – Default gw-type-prefix
 – E.164 Aliases (including Alias Static commands for CCM numbers)
 – Zone Prefix

Looking at the chart, we’ll need to match Step 1. Though it’s typically used to route calls to Remote Zones via a Location Request (LRQ), the gatekeeper can also generate an ACF at this stage if the Hop-off points to a Local Zone.

There isn’t much configuration needed to get this working; however, for your configuration to “stay working“, you would need to “hardcode” the signalling port used by your GK-Controlled Trunk (on Call Manager) to always use port 1720 as its Gateway Signalling Port.
This is quite important because CCM dynamically assigns port numbers for gateway signalling and for RAS.
These ports are set to 1720 and 1719 respectively on the gatekeeper by default.

If you don’t do this and the proctor resets your Trunks before grading (or maybe he/she even reboots the CCM box), then you’re screwed big-time 🙂

You can verify the Signalling and RAS ports CCM registered with on the gatekeeper with the show gatekeeper endpoints command:

GK-RTR#sh gatekeeper endpoints
CallSignalAddr  Port  RASSignalAddr   Port  Zone Name         Type    Flags
————— —– ————— —– ———         —-    —–    2789    4318  gk-zone1          VOIP-GW
    H323-ID: CCM_GKCT_1
    Voice Capacity Max.=  Avail.=  Current.= 0   1720   57634 gk-zone1          VOIP-GW
    H323-ID: CME_Trunk
    Voice Capacity Max.=  Avail.=  Current.= 0
Total number of active registrations = 2

As shown above, CME registers with 1720 by default, but CCM registers with 2789.
If CCM_GKCT trunk is reset, the port number would change to something totally different.
To configure CCM to use the port 1720 (signalling port) for a particular gatekeeper-controlled trunk through the CCM Service Parameters:

Navigate to the CCM Service Parameters and do a search for 1720. You’ll find the section under Clusterwide Parameters (Device – H323).
Change the name from None to the name of your trunk. In my case, my trunk is called CCM_GKCT.
Once this is done, you’ll notice that the port number for your CCM trunk would now be 1720 after you reset it.



We would now need to type in 3 commands to cater for the prefixes in HQ, BR1 and BR2.
 zone local gk-zone1
 gw-type-prefix 2* hopoff gk-zone1 gw ipaddr 1720
 gw-type-prefix 3* hopoff gk-zone1 gw ipaddr 1720
 gw-type-prefix 4* hopoff gk-zone1 gw ipaddr 1720
 no shutdown


Depending on whether we’ve been instructed to provide redundancy, we may need to include the secondary call-processing engine in the command for HQ and BR1 prefixes.
I’ve only got one call-manager running in my home lab, but here’s how you’ll configure it with two call-managers (assuming your second call-manager IP Address is

 gw-type-prefix 2* hopoff gk-zone1 gw ipaddr 1720 gw ipaddr 1720
 gw-type-prefix 3* hopoff gk-zone1 gw ipaddr 1720 gw ipaddr 1720


You can verify your configuration by using the show gatekeeper gw-type-prefix command (or by making calls ;-))


GK-RTR(config-gk)#do sh gatekeeper gw-type-prefix                         
Prefix: 2*    (Hopoff zone gk-zone1)
  Statically-configured gateways (not necessarily currently registered):
  Zone gk-zone1 master gateway list: CCM_GKCT_1

Prefix: 3*    (Hopoff zone gk-zone1)
  Statically-configured gateways (not necessarily currently registered):
  Zone gk-zone1 master gateway list: CCM_GKCT_1

Prefix: 4*    (Hopoff zone gk-zone1)
  Statically-configured gateways (not necessarily currently registered):
  Zone gk-zone1 master gateway list: CME_Trunk


PS: I’ve assumed your CME side has the following configuration:

dial-peer voice 1000 voip
 destination-pattern [23]…$
 session target ras
 incoming called-number .


Now, that’s a long post!!!!!