Peer to Peer calls over a GRE tunnel

Unanswered Question
May 27th, 2008

I have a GRE tunnel connection setup between to clients. I am tring to to call the remote office over a Comcast Cable connection. The other party hear a a ringtone but when he answers the phone we get dead air, this is for calling both ways. But when we use the FXO/FXS port using a analog phone we get 2 way communication.

I have this problem too.
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Michael Owuor Tue, 05/27/2008 - 05:43

Eric,

Double-check the routing here. Verify that a packet from the subnet that the phones on Cust_A's are on can find its way to the the subnet that the phones on Cust_B's are on.

It sounds like the routing is fine to and from the subnets that are used as the source addresses by the FXS/FXO ports, but not the subnets that the IP phones are in.

Regards,

Michael.

emaye135 Tue, 05/27/2008 - 07:18

We are connected via a GRE tunnel, while we have phones on each end registering on it own side, we can dial each other but no voice. Analog to digital work fine in both directions for voice. Do we have to be on the same subnet over the tunnel? Can you send configuration example for ip phones across a tunnel?

Michael Owuor Tue, 05/27/2008 - 07:59

You don't have to be on the same subnet over the tunnel. But there must be reachability between the two subnets one either side of the GRE tunnel. What evidence do you have that there is reachability between these two subnets? This is the reachability test whose success we'd like verified. One-way or no-way audio problems are commonly caused by routing issues, hence the request to have this ruled out.

The assumption I am making here to explain why call setup succeeds is that the CallManagers that are responsible for setting up the calls are not on the same subnet as their local phones, but can reach the remote subnets fine. Is this a correct assumption?

Regards,

Michael.

emaye135 Tue, 05/27/2008 - 09:57

Yes, we can reach both subnet across the tunnel using eigrp. Routing is good. Another addition is that we are not using our edge routers connecting that's connecting the tunnel as our voice routers. The voice routers on both sides are behind the edge routers connecting to the internet. Is this a factor. Does the edge routers need to be the voice routers in this scenario?

Michael Owuor Tue, 05/27/2008 - 10:15

Thanks for the update, Eric. For your setup to work, it is not a requirement that the edge routers need to be voice routers. In a VoIP network the physical location of the endpoints that need to communicate to each other matters little. They endpoints that must communicate with each other depend on the IP network to route the voice packets wrapped in IP and transmitted in the conversation. Sorry to harp on this little point, and I know you said routing is good, but what is the specific evidence you have that there is reachability between the two subnets where the IP phones are in? Traceroute? Ping?

When you have a call connected between two phones separated by the tunnel, hit the "I" button on each phone twice in rapid succession to allow you to view the statistical information about the call. Of interest here would be the packets transmitted and packets received. Are these values incrementing on both phones during the duration of the call?

Regards,

Michael.

emaye135 Thu, 05/29/2008 - 06:23

Do we need to have voice capable switches with switchport voice command below. Is the parameter actually required?

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch(config)# interface gigabitethernet0/1

Switch(config-if)# mls qos trust cos

Switch(config-if)# switchport voice vlan dot1p

Switch(config-if)# end

emaye135 Tue, 05/27/2008 - 07:57

We are connected via a GRE tunnel, while we have phones on each end registering on it own side, we can dial each other but no voice. Analog to digital work fine in both directions for voice. Do we have to be on the same subnet over the tunnel? Can you send configuration example for ip phones across a tunnel?

emaye135 Thu, 05/29/2008 - 07:41

Do we need to have voice capable switches with switchport voice command below. Is the parameter actually required?

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch(config)# interface gigabitethernet0/1

Switch(config-if)# mls qos trust cos

Switch(config-if)# switchport voice vlan dot1p

Switch(config-if)# end

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