Wolfgang Solfrank has explained the problem with router discovery
in `routed` in a way I can understand.
Let's assume that the configured preference of the interface is 5.
This gets converted to 0x80000005 through the use of the UNSIGN_PREF
macro. Later on, this value gets put into the PREF macro, which
compares it against the interface metric(s) (let's assume those
values to be 0 for now). Of course the 0x80000005, cast to int,
is much less than 0, so the clamping rule is triggered, which
gives us a value of 1. This is then converted via SIGN_PREF into
0x80000001 and put into the message. Certainly, this isn't what
was intended.
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