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！

1. weight
Prefer the path that has the largest weight.
思科的专有属性。

2. 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。

3. 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。

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是指那种老的协议，现在已经不再用了。

6. MED
If the origin codes are the same, prefer the path with the lowest Multi Exit Discriminator (MED 范围0到4,294,967,295).
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.
而且MED只在相同AS的情况下才对比，默认情况下不同AS的路由是不对比MED的。解决本AS域的入口路由选择。
另外MED的通告规则：
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.
优选对BGP下一跳具有最低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。

暂时更新到这，里面有解释的基本上是我常遇到的！