I need your help to ascertain why ISDN backup doesn't fire automatically when the primary leased line link goes down. As soon as I shut down the modem the ISDN line fires, how can I isolate the problem?
When you say the "link goes down," what OSI layer to do you mean? IP-level connectivity (Layer 3)? Layer 2 link? Layer 1 signal? What is the failover mechanism in the router set to trigger on?
It seems likely to me that you have failover set at a Layer other than the one you are looking expecting. Or, that Layer isn't behaving properly.
I suspect the router is set to respond to DSR/DTR on the modem and the modem isn't communicating that properly. In fact, shutting down the modem to generate a failover is convincing evidence to that effect.
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