What effects does double-NAT have on VoIP? What happens to the traffic in this type of situation and what conditions can occur as a result of it?

  • If your NAT's are SIP aware, none. (other than a microscopic amount of delay) – Ricky Aug 25 '14 at 19:47
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    @RickyBeam - What do you mean by SIP aware? Is that something that can be configured on all routers and modem/routers or are only some? – THE DOCTOR Aug 25 '14 at 20:04
  • "SIP aware" means the NAT service knows how to doctor SIP traffic. (see also: nat helper) Generally, there's nothing to configure, it does it or it doesn't. – Ricky Aug 25 '14 at 20:38
  • Are we speaking of SIP ALG? I've seen that feature cause a lot of trouble for VoIP traffic with call quality issues, dropped calls, etc. – THE DOCTOR Aug 25 '14 at 20:45
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 9 '17 at 2:01

As @Ricky Beam indicated, you should have no issues other than delay with fully-functional, SIP-aware NAT devices. This is known as ALG (Application Layer Gateway) on some lower-end network devices and SIP Fixup or SIP Inspection on different Cisco firewall platforms depending on software version.

Generally, the recommendation on not using ALG comes from poor implementation of either ALG itself or the client not putting its address into the SIP packets for it to be found and updated with the NAT'd address. On higher-end network devices, this generally works though you may run into a bug as I did in a recent question with an older Cisco PIX version.

Any type of ALG/SIP Inspection requires symetrically routing especially if implemented in a stateful firewall so the NAT'd addresses can get reversed on their way back to the client sitting inside the NAT device.

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