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WAN Fail Over & Load Balancing

April 25, 2013

I love it when things just work.

We have a remote location that runs on three T1 circuits.  It rarely goes down but rarely is not synonymous with “never”.  So a while back I configured a backup cellular data connection.  Not much bandwidth, mind you, but enough of a band-aid to hopefully keep folks working should it ever be needed.  It’s a pretty basic concept – you specify targets to probe over the WAN and rules for what happens when a target fails.  In my case I have two targets set – if they both fail in a row then the router switches to the secondary WAN interface (the Cellular modem).

WAN Probe Failure Alert

WAN Probe Failure Alert

Got the alert one evening at dinner that things seemed a little slow, but were working.  Upon checking the logs on the router I saw that the primary WAN / T1 was failing when trying to probe both targets and that the secondary WAN was now in use.

The cool thing was that once the T1 was fixed 30 minutes later, the router realized it’s probes were succeeding once again and it switched back to the primary WAN.  The rules I set said that the primary targets had to both have successful probes for 3 rounds before switching back.



From → Networking

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