CCIE SP — BGP 13条选路原则
If the next hop is unreachable, do not consider it.
如果下一跳不可达,是不会参加选举的!
Weight
Prefer the path that has the largest weight.
思科的专有属性,优选值高的路径,而且本地本地有效;
PS:Juniper没有这个概念;思科默认32768;华为对应的是PrefVal,默认是0;H3C类似华为是PrefVal,但默认是32768;
如下当两个设备同时发默认路由,默认DC优选h3c发的,但如果DC想优选huawei设备发来的默认路由(故障时才使用h3c的默认路由),那么就需要在h3c收到这条默认路由后把PrefVal 改成32768,让其bypass这个选路规则,用后面的规则如LP优选huawei发的:

Local Preference
If the routes have the same weight, use the route with the highest local preference. The local preference attribute only is local to the autonomous system and does not get passed to EBGP neighbors. The higher the local preference, the more preferred the route is.
默认情况下,从EBGP学来的路由local preference是100,解决本AS域出口路由选择。他不会把此参数传递给EBGP的邻居,优选值高的;
Originated
If the routes have the same local preference, prefer the route that was originated by BGP on this router.
优选从本路由器使发的路由,包括本地network,重分发和或IGP已有的路由,有BGP配置的聚合地址也包括在内。查看BGP路由表时可以看到有时路由前面带了一个“r”,说明在IGP中已经存在此路由了;
优先级为:default-originate > default-information-originate > network > redistribute > aggregate-address
完整阅读