While RFC 4861 specifies, that a cur hop limit of 0 must be parsed as "undefined" and therefore ignored, NetworkManager will set the hop limit of the interface to 0, leading to no IPv6-connectivity at all.

Steps to reproduce:
1. Install a router which announces a Cur Hop Limit of 0 (e.g. odhcpd, which is used in openWRT)
2. Connect a host with NetworkManager to this Interface
3. cat /proc/sys/net/ipv6/conf/<INTERFACE>/hop_limit

Expected result: 64 (or whatever was there before)

Actual result: 0

This Bug leads to all NetworkManager-Hosts behind most OpenWRT-Routers with missconfigured and unusable IPv6, therefore I set severity to major.

The Kernel-implementation uses a simple if to catch this [1], from my naive point of view, it should be easy to add this clause to [2].

1: https://github.com/torvalds/linux/blob/master/net/ipv6/ndisc.c#L1218
2: http://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/rdisc/nm-lndp-rdisc.c#n627