if the next hop is unreachable,do not consider it.
1.weight
prefer the path that has the largest weight
优先选择高权重值的路径
cisco私有属性
2.loacl preference
if the routes have the same weight,use the route with the highest loacl preference.
具有相同权重值的，将选取最高优先级得路径
the loacl preference attribute only is local to the autonomous system and does not get passed to EBGP neighbor
本地优先属性只在本自治系统有效，不会传递给EBGP邻居
3.originated
if the routes have same local preference,prefer the route that was originated by BGP on this router.
优选从本地路由器使发的路由，包括本地network，redistribute和IGP已有的路由，有BGP配置的聚合地址也包括在内。
4,AS Path
if no route was originated,prefer the route with the shortest AS path
5,origin code
if all paths are of the same as length,prefer the route with lowest origin code(IGP < EGP < incomplete)
EGP已经不在使用
6，MED
if the origin codes are the same,prefer the path with the lowest multi exit dicriminator(多路径入口，主要影响入站流量)
the MED is exchanged between ASs;however,the MED that comes into an AS does not leave.
the MED can be used to influence the outbound decision of another AS.the lower the MED,the more preferred the route.
MED只在相同AS的情况下才对比，默认情况下不同的AS路由是进行MED对比的。
MED通告规则：
1，ebgp在发布从ibgp学来的路由时，会清除MED的值。如果想强制通告，可以在宣告的路由上使用"set metric-type internal"
2,如果使用network或redistribute命令将来自IGP的路由通告到BGP中，那么BGP的MED是从IGP的metric导出的。
3，如果是aggregate-address命令注入路由，BGP的MED不被设置。
7,External paths over internal paths
if the MEDs are the same,prefer external paths over internal paths.
EBGP > confederation > IBGP
8.The path through the closest IGP neighbor
if they are still the same,prefer the path through the closest IGP neighbor
优先选取BGP下一条具有最低IGP度量值的路径
9. Multiple paths
Determine if multiple paths require installation in the routing table for BGP Multipath.
如果配置了“maximum-path N”。而且从同一个AS的对等体收到多条外部路由，侧可以将N条路由加入到路由表中，使EBGP负载分担。
N最大不能超过6；没有配置此命令是，默认是1。可以通过“sh ip bgp X.X.X.X”来查看最优的路径，这个最优是按接收时间来定的。
在这条最优路由转发到内部对等体之前，需要执行与“next-hop-self”一样的功能。
10. Prefer the path that was received first
When both paths are external, prefer the path that was received first (the oldest one)。
这可以使路由抖动重新收敛的影响降到最低，因为新的路径不会取代老的路径。
11. BGP Router ID
If they are still the same, prefer the path with the lowest BGP Router ID.
12. Cluster-ID
如果从路由反射器学到，优选最小Cluster-ID的路由。
13. Peer Adress
优选具有最低对等体地址的路由。