Impossible to run over 7 calls on SIP Trunk over VPN

Unanswered Question
Aug 14th, 2007

We have a VoIP SIP Trunk between a Cisco 2611XM and Cisco 2811 connected using IPsec tunnel. Using G729 codec in a 512 kbps link, we can run 5 to 6 simultaneous calls. Increasing to 7 calls bring all connected calls to crash, whereas we see that BW used doesn't overpass 350 kbps. Your help will be apreciated. Thx

I have this problem too.
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Paolo Bevilacqua Tue, 08/14/2007 - 07:42

Is this a direct p-t-p link? Does the 2611 have an AIM for VPN or it doing that in software ? What happens if you do the trunking without VPN ?

connect101 Tue, 08/14/2007 - 08:52

Bevilacqua, thanks for your prompt response.

It's not a p-t-p link, it passes over Internet. The 2611XM doesn't have AIM - VPN. Is the AIM essential for compressing to abtain more than 7 calls. Thanks.

Paolo Bevilacqua Thu, 08/16/2007 - 02:28


the AIM will not compress anything, but the 2611 doing IPsec encryption in software, may become quite busy. check CPU usage, also there is an encryption alg. that is better for software encryption, search for SEAL on CCO.

Also, how do you know that you have true 512 bandwidth across the internet ?

In any case try using a non-encrypted GRE tunnell and see how it goes.

Hope this helps, please rate post if it does!

connect101 Thu, 08/16/2007 - 06:50


The 512 bandwidth is guaranteed by ISP and FTP test confirm it. The CPU usage does'nt overpass 5%. We'll try non-encrypted GRE. I'll let you know.

Paolo Bevilacqua Thu, 08/16/2007 - 07:50


under sustained traffic, check the very first value that is CPU at interrupt level, 2% in the example below:

CPU utilization for five seconds: 2%/0%; one minute: 2%; five minutes: 1%

Chances are that for a non hardware accelerated VPN router, it will be higher.


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