VoIP with Router

Unanswered Question
Aug 30th, 2009


I have the following topology :

VoIP application (H.248) is running over a topology that is connected to a Router in Port 1 .

Port 2 of the Router is connecetd to the softswitch.

The question is how to configure the Router in such a way that the application will work :

wnen user A would like to talk with user B , User A will need to send destination with Mac B (user A & B are not on the same LAN) , the frame will be received on the router on port 1 and need to be transmitted in port 1 in the reverse direction to the network topology to reach user B.

how can we accomplish this?

I have this problem too.
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Giuseppe Larosa Sun, 08/30/2009 - 03:40

Hello Yoram,

VoIP uses IP routing as any other application.

the basic difference with web surfing is the following:

instead of asking a DNS server to resolve a URL to an ip address

a voip phone asks to the softswitch to call the phone number of userB.

softswitch consults its database and answers providing information about ip address of voip phone of userB.

Then the signalling process goes on, and the two phones build two UDP streams to carry the packetized voice packets using RTP.

so the router has just to route traffic and to allow signalling sessions typically TCP sessions between phones and the ip address of the server.

mac address of phoneB is used only by last router device to send the ip packet inside an ethernet frame to phone B.

phone A sends the packet with a destination MAC address = its default gateway mac address

Hope to help


yoram12345 Sun, 08/30/2009 - 03:44

hi Giuseppe,

Correct but my problem is the following :

since the phone are not connected on teh same LAN and teh connectivity need to be done by the Router , how will router send the frame in the opposite direction through the same link .

Generaly the router will not send the frame back and that is what i need



Giuseppe Larosa Sun, 08/30/2009 - 05:06

Hello Yoram,

when we look at a phone conversation in terms of IP flows we can see there are multiple IP flows with different IP addresses that are used:


(phoneA_ipaddr, server_ipaddr)

one or more actually

(phoneB_ipaddr, server_ipaddr)

one or more actually

call setup (phoneA_ipaddr,phoneB_ipaddr)

all the above are usually TCP sessions

bearer streams that carry voice


if so the router is never in the need to send back the same packet out the interface in which it is received.

packets with IP SA,DA (phoneA_ipaddr,phoneB_ipaddr) are routed directly without going via the softswitch.

if the softswitch wants to be in the middle of media streams the following happens

call leg 1:

phoneA_ipaddr, server_ipaddr

call leg2:

server_ipaddr, phone B_ipaddr

this allows the router to work well

Hope to help


yoram12345 Sun, 08/30/2009 - 10:52

Hi Giuseppe ,

The thing is that the network which is connected to port 1 of the router is MPLS,and so phone A is connected to port 1 via tunnel A .

Phone B is connecte also to router port 1 via tunnel B.

due to split orizon packet or frame that goes from phone A will never go to tunnel B.

And so all the frames goes to the router that need to pass them in reverse direction through the same port 1 .

Now the frame can go through tunnel B to phone B.

So the question is how to cause the router to tranmsit the frames back

hope i explained myself



Giuseppe Larosa Sun, 08/30/2009 - 11:10

Hello Yoram,

it is not clear for me.

Attach a network diagram and part of configurations of your router.

split-horizon is a concept that relates to:

routing updates

and works when a device should advertise a route out the same interface where it has learned it.

Here we are speaking of voip packets so I don't see why split horizon should play a role here.

Besides this if there are two tunnels they are two distinct layer3 interfaces so split-horizon should not apply to them.

Difficult to say more without some details.

remove sensitive information like public ip addresses and passwords

Hope to help


yoram12345 Sun, 08/30/2009 - 11:15

Hi Giuseppe ,

The MPLS network is not cisco , its the facility of the customer that i need to use .that's how it works :

from all phones u have tunnels to the same point of the router.

The router should be the one who shoud switch the packet in reverse direction .

seems strange but that's how this works.




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