VoIP Setup- Routers and Switches

Having more than one NAT/router in a network can create issues that will affect your VoIP connection.  You may experience issues right away, or they may develop sporadically over a period of time.  These issues can include one-way audio and calls dropping for no apparent reason.  Setting up your network correctly can be beneficial for your VoIP quality and connection, as well as the overall stability of your LAN.  We have added information on best practices, including how to configure your second wireless router after adding a VoIP Linksys  RT31P2 ATA/router.

Routers, switches and other Devices:

Starting with the Internet connection you will usually find a DSL modem, cable modem or another firewall/router device.  These devices can have both firewall capabilities and be NAT/routers, supplying DHCP addresses to the internal LAN devices.  The problem for VoIP is that they may use an unfriendly NAT transversal or SPI (stateful packet inspection), preventing the voice packets from making it back to the ATA correctly.  (Netgear routers use both a problematic NAT and SPI.  You may need to place the ATA into a DMZ zone on a Netgear.

In most cases, if more than one NAT/router is between the ATA and the Internet, this double NAT issue will create connection problems and one-way audio.  Add to that the fact that double NAT is not considered good network practice, you will want to make sure to limit the amount of devices supplying DHCP. 

Many of our featured business VoIP providers below include a free setup and router with their service plans:

• More than one device with DHCP enabled between the ATA and the public IP.
• A device that uses NAT unfriendly SIP or SPI.
• Firewalls blocking the UDP packets needed for the ATA.

Most DSL modems act as a routers for the internal LAN, while most cable modems do not.  (This is the primary reason why a cable VoIP installation will be much easier than a DSL install.)   Some DSL modems and certain types of NAT/routers can use a SIP unfriendly NAT.  The result can be one way-audio, where the other party may hear you, but you cannot hear any incoming audio.  This is an issue caused by the return voice packets stopping at the router, firewall or double NAT router and never finding their way back to the ATA.  If you suspect this, then:

• Make your DSL a bridge and authenticate on the router or ATA/router using PPPoE.  If connecting a separate IAD device, then connect it directly to the router.
• Change any second NAT/routers to a switch and disable DHCP.

If you need to add a wireless or second router to your LAN and are using a Linksys router/ATA, then the best configuration would be as follows. 

• Connect the Linksys/ATA (address 192.168.15.1) to the modem using the Internet port of the ATA.
• Connect a PC to your second router (address 192.168.1.1) and access the internal setup pages.  Disable DHCP and save settings.
• Connect the second router via an Ethernet cable from a numbered port on the ATA VoIP router to the second router's Uplink port (if this second router does not have an uplink port then try a regular port).

• How to connect a Linksys wireless router to another router acting as a DHCP server.
• How to connect a D-Link wireless router to another router acting as a DHCP server.
• How to connect a Netgear wireless router to another router acting as a DHCP server.
• How to connect a Belkin wireless router to another router acting as a DHCP server.

One-way Audio:

VoIP protocols often will not function correctly in networks that include some types of NAT transversal or firewall components. The result is usually that calls can have one-way audio or fail.

One-way audio is usually the result from a double NAT issue or an unfriendly NAT transversal being used on the network.  VoIP itself presents a problem for NAT, in that the normal protocols it uses deal with the signaling and establishing of the connection.  These packets are generally identified by the NAT/router with the ATA device and will be forwarded correctly.  But, once the connection is established, the audio, which is an entirely different protocol and can be on a port initiated from an outside source, confuses the router.  It does not recognize the incoming packets as belonging to the ATA, and so they get dropped. 

To alleviate some of these NAT issues some VoIP providers run Session Border Control servers.  These devices both handle traffic and shape the packets to  get past NAT transversal and firewalls.

The first step would be to try to limit the devices between the ATA and the Internet.  To do this connect the ATA directly to the first device, which would usually be a modem.  (You may have to do a power cycle of the modem to get the VoIP ATA to connect. more)  If after the ATA connects and establishes dial tone you have two way audio, then you know that it works at that point in the network and you will need to locate the device that is blocking the transmission further down line (usually a second router). 

If the VoIP ATA does not establish two-way audio connected directly to the modem (first device), then look at these possible issues and solutions:

• A DSL modem that uses an unfriendly NAT transversal and does not pass VoIP protocols easily.  In this case you should consider bridging the modem and configuring the ATA/router or a VoIP protocol friendly router to use PPPoE, with username and password. 
• Determine where the public IP address is sitting and if possible configure the network so the ATA/router or IAD receives a public IP address.
• Configure any firewalls to allow the correct ports needed to pass directly to the ATA.
• Place the ATA VoIP device in a DMZ or perimeter zone, where it will sit outside the firewall.

Step by Step Screenshots on Bridging a Westell 6100