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MGCP fractional PRI and 6608.

Having a problem with 6608 and T1 PRI (fractional)

6608-----backtoback----7200 (port adapter with t1/e1)

I am configuring only 6 channels on the 6608 and the 7200.

Enable status poll is checked.

Channel selection order is Bottom Up on 6608. Channel selection order on 7200 is set to ascending. So there is no glare.

BChannel maintenance 1 is set to

<gateway name> = 0000 0011 1111 1111 1111 1111

When I call a number in PSTN simulator (7200) from the IP Phone, calls are successfully placed, except that 6608 always uses channel 23 instead of channel 6.

Why does this happen ?

I replaced the 6608 with a Cisco 2600 router and when it places the call, it uses channel 6.



Re: MGCP fractional PRI and 6608.

Hi Sankar,

Which CCM version and gateway load is using the blade?

Do you have CCM detailed trace for that call?

Also the debug isdn q931 for the 7200.

In the Perfmon what is the B channel status for the 6608 PRI (Cisco MGCP PRI Device)?

The 2600 also MGCP?

If you generate lets say 7 calls in fact it follows the order for channel selection and drop call #7?

Just as reference:


CallManager needs support for MGCP fractional PRI

Re: MGCP fractional PRI and 6608.

CCM version 3.3.3

2600 is also MGCP. (with bchannel maintenance configured for 6 channels)

I keep getting a syslog on the 7200, saying,

"requested ISDN channel not available" - obviously because the 6608 is trying to send a call on channel 23, while the other end router has only channels 1 through 6 defined..

I will try to get the debugs here soon..

Thanks for your help!

Re: MGCP fractional PRI and 6608.

I just ran into this problem and here is the solution.. You must use the opposite of the telco.. Meaning in your configuration your 6608 configuration in CM needs to be Top Down.. I just tested this and it works fine...

Re: MGCP fractional PRI and 6608.


Quoting from Callmanager help page for channel selection order...

"Channel Selection Order

Choose the order in which channels or ports are enabled from first (lowest number port) to last (highest number port), or from last to first.

Valid entries include TOP_DOWN (first to last) or BOTTOM_UP (last to first). If you are not sure which port order to use, choose TOP_DOWN. "

So if I understand this correctly, TopDown is 1 to 23 and Bottom up is 23 to 1.

If the 6608 is set to Bottomup, to avoid glare, I set the other side router to ascending (1 to 23).

Correct me if I am wrong on this ?


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