IPv6 Network - Neighbor Unreachability Detection RFC 2461 also specifies a procedure for neighbor unreachability detection. IPv6 hosts and routers actively track whether their neighbors are reachable. They do this by periodically sending neighbor discovery messages directly to the neighbor. If the neighbor answers, it’s reachable; if it doesn’t, there must be some kind of problem, and the system will discard the neighbor’s MAC address and try a regular multicast neighbor discovery procedure. This allows IPv6 systems to detect dead neighbors and neighbors that change their MAC address. But it’s most useful to detect dead routers. On a subnet with more than one router, a host can simply install a default route toward another router when the router that is has been using becomes unreachable. Windows XP, Linux, MacOS, and FreeBSD all take the situation where a router loses its IPv6 address and no longer runs IPv6 in stride and switch over to another router without incident. However, turning off the active router has much more severe effects: at the very least, ongoing downloads stall for a while, and in some cases, the session breaks. I have no explanation for this difference in behavior.
