三、看会不会在一条链路down 之后走另一条链路：
1、如果我把R2中的S0/0接口down 掉：

R2(config)#int s0/0

R2(config-if)#shut

 
2、再看一下R4和R5上的走向：
R4#traceroute ip

 
Target IP address: 1.1.1.1    

Source address: 192.168.24.4

Numeric display [n]:

Timeout in seconds [3]:

Probe count [3]:

Minimum Time to Live [1]:

Maximum Time to Live [30]:

Port Number [33434]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Type escape sequence to abort.

Tracing the route to 1.1.1.1

 
  1 192.168.34.3 56 msec 24 msec 36 msec

  2 192.168.13.1 148 msec *  112 msec   （走到右边链路去了。还是能通。）

 
R5#traceroute 1.1.1.1

 
Type escape sequence to abort.

Tracing the route to 1.1.1.1

 
  1 192.168.45.4 92 msec 92 msec 76 msec

  2 192.168.24.2 92 msec 112 msec 64 msec

  3 192.168.24.4 92 msec 108 msec 84 msec

  4 192.168.34.3 124 msec 120 msec 108 msec

  5 192.168.13.1 152 msec *  172 msec 发现192.168.12.0不通，绕了回来再走右边，还能通。

 
 
3、把R2上的s0/0起来，把s0/1  shutdown ,看一下R4和R5到1.1.1.1有什么不同：
R2(config-if)#int s0/0

R2(config-if)#no shut

R2(config-if)#int s0/1

R2(config-if)#shutdown

 
R4#traceroute ip

 
Target IP address: 1.1.1.1

Source address: 192.168.24.4

Numeric display [n]:

Timeout in seconds [3]:

Probe count [3]:

Minimum Time to Live [1]:

Maximum Time to Live [30]:

Port Number [33434]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Type escape sequence to abort.

Tracing the route to 1.1.1.1

 
  1  *  *  *

  2  *  *  *

  3  *  *  *

  4  *  *  *

  5  *  *  *

  6  *  *  *

  7  *  *  *

  8                        （R4到1.1.1.1不通了）

 
R5#traceroute 1.1.1.1

 
Type escape sequence to abort.

Tracing the route to 1.1.1.1

 
  1 192.168.45.4 152 msec 68 msec 64 msec

  2 192.168.34.3 92 msec 64 msec 68 msec

  3 192.168.13.1 132 msec *  120 msec         （对R5没影响）

 
4、我再把R4的s0/1  端口起来，serial口是不会有这种情况，但像实际环境中，如帧中继网络和以太网交换机都会有这种情况，一端down ，另一端不会down.那么我现在通过no keepalive来把S0/1 up起来：
 
R4(config-if)#no keepalive

R4(config-if)#do sho ip int bri

Interface                  IP-Address      OK? Method Status                Protocol

Serial0/0                  192.168.34.4    YES NVRAM  up                    up     

Serial0/1                  192.168.24.4    YES NVRAM  up                    up     

Serial0/2                  192.168.45.4    YES NVRAM  up                    up     

Serial0/3                  192.168.46.4    YES NVRAM  up                    up

 
5、现在我再在R5上traceroute:
R5#traceroute 1.1.1.1

 
Type escape sequence to abort.

Tracing the route to 1.1.1.1

 
  1 192.168.45.4 124 msec 96 msec 72 msec

  2  *  *  *

  3  *  *  *

  4  *  *  *

  5  *  *  *

  6  *  *  *

  7  *  *  *

  8  *  *  *

 
R5#ping 1.1.1.1

 
Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

 
   终于不通了，因为它到R4时，R4给它的是up，它却应用策略路由走下去，但却发现不通了。