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Another update:

This problem only occurs when throttling is enabled in the direction of LNS --> CPE; if throttling is enabled the other way (CPE --> LNS), it does not cause any IPv6 packets to be dropped.

I've had a look through tbf.c, ppp.c and l2tpns.c; from what I can tell, the code which handles the throttling is near-enough the same for both directions (processipv6in and processipv6out) - there must be something direction-specific which is causing this issue.

Any ideas ?