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Redundancy and availability for simple gateway circuits

How to make sure that these simple gateway circuits still have redundancy and high availability.

You may have some little-used FXS-type analog circuits in your network that function as gateways between the VoIP network and the PSTN. While the trunks are low-capacity (only one call at a time) and relatively cheap, you still may have a need for redundancy or high availability on these trunks. Redundancy and high availability requirements usually demand two physical routers. So how do you connect them to a single POTS line?

The answer should probably be about 69 cents from Wal-Mart, maybe a dollar and a half nowadays. The same "line splitters" you use at home to have multiple phones and answering machines connected to the same phone jack, can be used to connect two routers to the same POTS line. It's very simple and easy to cable. But how do you configure the routers?

Although there may be a number of ways to do this, the simplest is probably to configure the routers' FXS ports to specify exactly how many rings they will wait before answering. Then set the primary router to answer in, say, 1 ring, and the backup router to answer in 3 rings. Obviously, as long as one is less than the other, it should answer first.

Thomas Alexander Lancaster IV is a consultant and author with over 10 years experience in the networking industry, focused on Internet infrastructure.

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