consider only （synchronized）routers with no AS loops and a vaild next hop ，and then：

1.prefer hightest weight（local to router）

2.prefer hightest local preference （global within AS）

3.prefer router originated by the local router （next hop = 0.0.0.0）

4.prefer shortest AS path

5.prefer lowest origin code （IGP>EGP>incomplete）

6.prefer lowest MED（exchanged between autonomous systems）

7.prefer EBGP path over IBGP path

8.prefer the path through the closest IGP neighbor

9.如果配置了maximum-path【ibgp】，如果存在多条等价的路径，会插入多条路径

10.prefer oldest route for EBGP paths

11.prefer the path with lowest neighbor BGP router ID

12.多条路径的始发路由器ID相同，那么选择CLUSTER_LIST长度短的

13.prefer the path with the lowest neighbor IP address