路由决策
当neighbour的状态发生变化的时候，会影响路由, rfc：www.rfc-editor.org/rfc/rfc4191中有相关描述
3.1 Conceptual Data Structures for Hosts --- paragraph 2
Type B hosts use a Default Router List augmented with preference
values, but ignore all Route Information Options. They use the
Default Router Preference value in the Router Advertisement header.
They ignore Route Information Options.
3.2 Conceptual Sending Algorithm for Hosts --- paragraph 2
When a type B host does next-hop determination and consults its
Default Router List, it primarily prefers reachable routers over
non-reachable routers and secondarily uses the router preference
values.
3.5. Router Reachability Probing
When a host avoids using any non-reachable router X and instead sends
a data packet to another router Y, and the host would have used
router X if router X were reachable, then the host SHOULD probe each
such router X's reachability by sending a single Neighbor Solicitation to that router's address.
上述理解为，当用默认路由的时候，会先看路由的可达性，然后再比较优先级。可达性时邻居发现（Neighbour Discovery）机制提供的。
邻居发现
rfc:datatracker.ietf.org/doc/html/rf…
neighbour discovery mechanism
datatracker.ietf.org/doc/html/rf…
此rfc比较容易懂，个人建议重点阅读章节：第5章、6.1节、6.3节和第7章
