Howto:Fault tolerance setup with three ISDN BRI lines

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As known, when using an IP800, an ISDN fault tolerance scenario can be realized. This is possible because an IP800 has the possibility to switch two ISDN BRI lines to a standby PBX (Gateway) in case of fault.

But what if a customer has three BRI lines?

No problem, here is how to deal this using still using just two IP800.

Applies To

This information applies to

  • IP800

Problem Details

IP800 has just two relays and therefore just two BRI interfaces can be switched, but you can use the second unit even in normal operation.

Just add some simple routings and wire the setup as show in the following picture:


Image:FT01.png

The blue lines represent the active ISDN lines, the red ones cabling waiting for action.

In normal operation the GW1 (the active PBX) will handle line 1 and 2 while the standby GW work with line 3 and via the network communicate with the GW1 (and active PBX).

The following ISDN lines will terminate on the Gateway/Port:

Line 1: GW1 – Tel1

Line 2: GW1 – Tel3

Line 3: GW2 – Tel3

In case of failure of the GW1 the following setup is given:


Image:FT02.png

The following ISDN lines will terminate on the Gateway/Port:

Line 1: GW2 – Tel1

Line 2: GW2 – PPP

Line 3: GW2 – Tel3


If instead standby GW fails, the following setup is automatically given:


Image:FT03.png

The following ISDN lines will terminate on the Gateway/Port:

Line 1: GW1 – Tel1

Line 2: GW1 – Tel3

Line 3: GW1 – PPP

Even if both gateways fail there is no shortcut, because the electrical situation will be the following:


Image:FT04.png

The lines will end to the following (not working) port.

Line 1: GW2 – Tel1

Line 2: GW2 – PPP

Line 3: GW1 – PPP

Note: The article describes the hardware wiring, the logical setup in the routing table is not part of this description, but follows the hardware design and is simple.

Note: You need on each Gateway 3 x BRI and 6 x DSP license.

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