OSPF 和 ISIS 使用 maximum-paths 命令指定 SPF 算法将考虑的最大等价路径数。默认情况下，它设置为 4，因此默认情况下，每个目的地最多考虑 4 条等价路径。

您可以使用show ip protocols命令查看每个协议的最大路径：

Switch#sh ip protocols 
*** IP Routing is NSF aware ***

Routing Protocol is "application"
  Sending updates every 0 seconds
  Invalid after 0 seconds, hold down 0, flushed after 0
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Maximum path: 32
  Routing for Networks:
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 4)

Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 1.1.1.1
  Number of areas in this router is 0. 0 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)

Routing Protocol is "isis"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Redistributing: isis
  Address Summarization:
    None
  Maximum path: 4
  Routing for Networks:
    GigabitEthernet0/1
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 115)

算法运行时，会看到两条等价路由，但是有一个tie-breaker（路由器ID，见这个问题），所以会选择一个作为最终结果。由于您可以控制路由器 ID（或者实际上也可以控制链路成本），因此您可以强制此选择采用任何一种方式。