路由器AR1-3的loopback ip分别为1.1.1.1、2.2.2.2、3.3.3.3,通过network发布
1. 默认情况下AR1查看2.2.2.2的路由，默认情况下，cost为1
<AR1>dis ip routing-table 2.2.2.2
Route Flags: R - relay, D - download to fib
---------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
2.2.2.2/32 OSPF 10 1 D 10.10.12.2 GigabitEthernet0/0/0
默认情况下AR2查看1.1.1.1的路由，默认情况下，cost为1
<AR2>dis ip routing-table 1.1.1.1
Route Flags: R - relay, D - download to fib
---------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
1.1.1.1/32 OSPF 10 1 D 10.10.12.1 GigabitEthernet0/0/0
2. 在AR1的接口配置cost为10
[AR1-GigabitEthernet0/0/0]ospf cost 10
#
查看AR1上2.2.2.2的路由，Cost已变为10.
[AR1-GigabitEthernet0/0/0]dis ip ro 2.2.2.2
Route Flags: R - relay, D - download to fib
---------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
2.2.2.2/32 OSPF 10 10 D 10.10.12.2 GigabitEthernet0/0/0
3. 在AR2上的接口配置cost为20
[AR2-GigabitEthernet0/0/0]ospf cost 20
查看AR2上1.1.1.1的路由，Cost已变为20.
[AR2-GigabitEthernet0/0/0]dis ip ro 1.1.1.1
Route Flags: R - relay, D - download to fib
---------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
1.1.1.1/32 OSPF 10 20 D 10.10.12.1 GigabitEthernet0/0/0
结论
OSPF COST跟收到的路由接口COST有关，对于AR1来说，AR2的路由从AR1的接口G0/0/0进入，因此AR1的G0/0/0的COST影响AR1收到AR2的路由。
即“路由和流量的方向是相反的”，AR1收到AR2的2.2.2.2的路由是AR2通告过来的，从AR2(20)--->(10)AR1，受AR1的入接口的cost影响，AR1学习到这条路由是为了访问AR2的2.2.2.2，访问路径是AR1--->AR2，因此路由和流量的方向是相反的。