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Donal Bray

Cisco to Cisco VPN. VOIP Issue

Hi All. 

I have a client and they wanted me to set up a site to site VPN for 2 offices. To do this I decided to use 2 cisco 867VAE-K9 units. I have most of it working byt we are coming up against a problem and I am a bit out of my depth. There are also 2 PBX's that were installed by Eircom (Irish telecoms provider) These are their from long before I became involved. These are set up with a remote office funtion between the 2 pbx units so that if Person in office A wants to ring extension in office B they dial the extension directly and the PBX in office A routes to PBX in office B over IP this is only half working when they dial the extension rings but the caller cannot hear the recipient but the recipient can hear the caller this happens both ways eircom have been on site and tell me their end of things are fine. I am attaching running configs for both router and would really appreciate any help. 




Hi Donal, do you know if you have a Smartnet contract with this devices?

I don't know..
We just purchased from our cisco distributor. We generally don't need Cisco support.

What is involved in getting one.

We were going to switch to LANCOM devices if we couldn't get the Cisco to work.

We have done that before because we needed solution.

Can you direct me to best practice in this situation.

Call me if you need to




Donal Bray 087 6185266


I hope we can we help you out here!  Please note that having a Service Contract is good for you not only in terms of the current issue you are facing, but because predicting from any future hardware or Software problems. The Cisco Service Contract  ensures you have adequate hardware/Software replacement upgrades at the time is needed. Ferdenand might have something to elaborate on this specific platform, however, my general recommendation would be to protect your investment what so ever its with Cisco smart contracts that is also affordable to get.


Now, Coming to your issue, your Config of both routers seems OK, but its missing two important things:

1- You need to set the correct MTU and MSS Value on both routers. this should eleminate any cause of drops due to fragmentation and hence impact your VOIP between Sites.

Please set the MTU Value on both router dialer interfaces to a value of 1439.

Please Set the tcp MSS value on the Vlan 1 interface of both routers to 1399 on both routers.

2- Check your VOIP session after the modifications, if its still there, then I would recommend having Quality of Service in place to ensure your SIP gets the required priority when there is a congestion. this ensures your VOIP gets the right QoS treatment if there is Bandwidth usage at the DSL lines.


Let us know your feedback,




Hi Mohamed 

Thanks for the input I have changed the MSS and MTU values I have spoken to the client and the issue still persists. 



Hi Donal send me an email at and ill try to help you out. Hope to hear from you soon!