With Xifos (and possibly previous ISP's), if I didn't disconnect "cleanly", for example, I yanked the power cable out of my router, I'd have difficulties reconnecting for anything up to 30-60 minutes. pppd will just cycle round and round, failing to reconnect each time. Though, a quick call (if Xifos ever answered) reveals that the issue was a stale session...as soon as they removed it, the connection come up instantly.
If this were to be your troubles, it could be made worse by sync loss, with your session still sitting open on the radius, causing issues re-authenticating after your line retrains.
Whenever it happened to me (upgrading router firmware usually), I'd just pace around the house while waiting for coffee to brew, worrying that I may never see the net again!
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