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SIP trunk alternative routing after receiving 486 Busy Here

Hello,

I've 2 SIP T.38 fax servers running in redundancy mode (Xmedius SP) : server1 and server2

On my CUCM 7.1.5 I've created 2 SIP trunks, one for each server IP address.

I've added these two trunks into a route group with a top-down distribution. Also tried circular.

The problem is when I have all my T.38 channels busy on server1 and free channels on server2, if a SIP INVITE is sent to server1, answer is 486 Busy Here. And that's all.

I would like the SIP INVITE to be redirected to the next SIP trunk defined in route group.

Is there something I've missed in the configuration or is it a SIP limitation ?

Thanks.

1 Accepted Solution

Accepted Solutions

Hi Jean,

What's the value of the Cisco CallManager service parameter 'Stop Routing on User Busy Flag' ? If set to 'True', can you try setting it to 'False', test the call and check if it makes any difference in the call flow ?

Thanks,

Sriram

Please rate helpful posts !

View solution in original post

6 Replies 6

Daniele Giordano
Collaborator
Collaborator

Try to add this command in global configuration:
router(conf)# voice hunt user-busy

Regards.

I tried router(conf)# voice hunt user-busy , but it doesn't work.

Voice router is not involved because it is only SIP signaling at the beginning between CUCM and the fax server. And voice router is only a MGCP Gateway so call control is onto CUCM.

CUCM  ---- INVITE ---->  FAX_SERVER

CUCM  <---- TRYING ----  FAX_SERVER

CUCM  <---- 486 BUSY HERE ----  FAX_SERVER

CUCM  ---- ACK ---->  FAX_SERVER

What is strange is I can see SUBSCRIBE requests from CUCM to both fax server IP addresses before the SIP INVITE on the first fax server.

Fax servers answers to this subscrive are : 481 Call Does Not Exist

So CUCM is aware the route is made of 2 IP addresses. But only for Subscribe.

Any other idea ?

Thanks,

Hi Jean,

What's the value of the Cisco CallManager service parameter 'Stop Routing on User Busy Flag' ? If set to 'True', can you try setting it to 'False', test the call and check if it makes any difference in the call flow ?

Thanks,

Sriram

Please rate helpful posts !

Hello srsivara,

I wasn't very optimistic because "Stop Routing on User Busy Flag" is indicated for intercluster trunk calls.

But it works perfectly for SIP trunks also . Many thanks !

Due to the fact this is a clusterwide parameter, I hope this change from True to False will not create other strange behaviours...

It would be a good idea to include this parameter in the Route List Detail or Route Group Configuration when using SIP Trunks.

Regards,

Hi Jean,

Good to know it worked !

1. The service parameter description threw me off too yesterday, but I remember working with that parameter in a similar situation a while back. I will discuss with the development team to update the service parameter description

2. Setting that service parameter to False might cause extra call routing attempts by CallManager. Here's an example where it doesn't help :

Phone in CCM --- Route pattern --- Route List (gw1 , gw2) --- pstn --- cell

If cell phone is busy, and telco sends cause code 17 or 486 (user busy) through gw1, CallManager will try routing the call again through gw2. In this case, the attempt is just not needed (assuming that the cell phone doesn't become available in the next couple of seconds), but CallManager does it, as the "Stop Routing on User Busy Flag" flag is set to False. Even in this case, things don't break - just additional processing on the part of CallManager - so you should be fine

3. "It would be a good idea to include this parameter in the Route List Detail or Route Group Configuration when using SIP Trunks" - that's a great idea. I would request you to talk to your Cisco Account Team, to see if they can initiate a discussion with the dev team and find out if they can entertain this enhancement request.

- Sriram

Please rate helpful posts !

any updates on this great idea?

I do not see the different on CUCM 14.0

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