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VPN/Voice problems

We have a remote site running a 1760 as its edge router. The 1760 also connects the site back to a Concentrator 3000 at our home office using a site-to-site VPN connection. The VPN is only for intranet applications such as file sharing as well as VOIP. No Internet bound traffic enters the tunnel.

The main office houses CCM gear and our handsets communicate with the CCM through the VPN link. The problem is with VOIP operations at the remote office. If multiple phone calls are attempted, all but the first are hit with jitter, drops, and delay. Sometimes, the effects come and go throughout a conversation and at other times, the phones are useless. At first the thought was that this was a problem with load on the leased lines, but that turns out not to be the case. It obviously gets worse with load, but it also happens when there's little load on either end.

Does anyone have any suggestions on what I can look at to correct this problem. Please let me know if further information is required.

Thanks,

Tom

1 REPLY

Re: VPN/Voice problems

Hello,

what is the bandwidth available on the leased lines? You could have jitter problems caused by large data packets blocking voice packets. Link fragmentation and interleaving (LFI) would help then.

How did you check load on the lines? "show interface" might not reveal anything, because the load interval is at least 30 seconds, but VoIP degradation is possible even with 1 second load peaks.

Enable LLQ for VoIP at the leased lines in any case, you WILL need it.

Do not forget about LAN switches, priority queueing is recommended there as well.

And implement call admission control to limit the number of concurrent VoIP connections to what your priority queue can handle.

So in brief:

1) if bandwidth anywhere lower than 768 kbps the use LFI

2) use LLQ on every interface VoIP traffic is present ("auto qos voip" is a good starting point.)

3) priority queueing for VoIP in LAN switches

4) Call admission control.

Hope this helps! Please rate all posts.

Regards, Martin

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