Send source link-layer address option in router advertisements.

Send source link-layer address option in router advertisements.

With this, hosts immediately learn the layer 2 (i.e. ethernet mac)
address of their default router and don't need to do another round
trip.

It also turns out that apple devices (macOS & iOS) install the default
route as what they call "interface scoped" if a DNS option is present
and the source link-layer address option is absent. This effectively
makes the default route unusable.

Problem with fruit devices tracked down & diff by Ryan Vogt (rvogt.ca
AT gmail), thanks!

OK sthen, bket