网络拓扑：
两台路由器通过串行接口S1/1连接，中间配置IP子网为 12.1.1.0/24，R1为 .1 ，R2为 .2
模拟环境使用EVE仿真环境，加载了IOL模拟器，感谢仿真环境和模拟器的开发者。
故障描述：
管理员做了如下配置，但R1和R2之间却无法ping通，甚至自己ping自己也无法通讯。
R1
configure terminal
interface s1/1
encapsulation ppp
ip address 12.1.1.1 255.255.255.0
no shutdown
end
R2
configure terminal
interface s1/1
ip address 12.1.1.2 255.255.255.0
no shutdown
故障现象：
R1#ping 12.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.1.1.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R1#ping 12.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.1.1.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
故障诊断：
??? 方法一：通过查看基本配置。
R1和R2的接口下一个配置了封装类型为PPP，另外一个却没有配置，而IOS操作系统下，串行接口默认的封装类型为HDLC，这就好比两个人在使用不同国家的语言聊天，根本互相听不懂啊。
??? 结论为，链路两端接口数据封装类型不一致导致故障。但是这种方法太过武断，需要有丰富的实践操作经验和扎实的理论基础支撑，并不推荐哦。经验老道的工程师们，可以在此基础上做进一步诊断，确定故障原因。
??? 方法二：通过log做出诊断
??? 对于有一定经验的工程师来说，其实在前面的配置过程中通过查看IOS操作系统给出的反馈就可以得出结论，看哪里呢？
R1#configure terminal
Enter configuration commands, one per line.? End with CNTL/Z.
R1(config)#int s1/1
R1(config-if)#encapsulation ppp
R1(config-if)#ip add 12.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#
*May? 4 08:19:21.872: %LINK-3-UPDOWN: Interface Serial1/1, changed state to up
R1(config-if)#end
R1#
是的，细心的人会注意到，这个位置少了点什么，所谓事物反常即为妖，那么究竟少了什么呢？我们来看一个接口开启之后应该出现些什么。
R1(config)#int e0/0
R1(config-if)#no shutdown
*May? 4 08:31:02.560: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
R1(config-if)#
*May? 4 08:31:03.564: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up
R1(config-if)#
不仅会有一个接口开启的log，同时正常情况下链路层也应该工作正常，因此还会出现一个链路层up的log。也就是说，实际上，R1的S1/1接口网络接口层并没有正常工作；
R2(config)#int s1/1
R2(config-if)#ip add 12.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#end
R2#
*May? 4 08:20:55.922: %SYS-5-CONFIG_I: Configured from console by console
R2#
*May? 4 08:20:57.488: %LINK-3-UPDOWN: Interface Serial1/1, changed state to up
*May? 4 08:20:58.496: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up
R2#
*May? 4 08:21:23.792: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to down
再来看R2上面的log，接口链路层UP后又down了，这说明什么问题呢？说明R2的S1/1接口网络接口层也没有正常工作；
接下来验证我们的猜测：网络接口层工作不正常
R1#show ip interface brief s1/1
Interface????????????????? IP-Address????? OK??? Method?????? Status ????????????? Protocol
Serial1/1????????????????? 12.1.1.1????????? YES????? manual???????? up???????????????? ?? down???
R2#show ip interface brief s1/1
Interface????????????????? IP-Address????? OK???? Method????? Status???????????? ?? Protocol
Serial1/1????????????????? 12.1.1.2????????? YES?????? manual??????? up ????????????????? down
R1#show interfaces s1/1
Serial1/1 is up, line protocol is down
?? Hardware is M4T
?? Internet address is 12.1.1.1/24
?? MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
????? reliability 255/255, txload 1/255, rxload 1/255
?? Encapsulation PPP, LCP Closed, crc 16, loopback not set
R2#show interfaces s1/1
Serial1/1 is up, line protocol is down
?? Hardware is M4T
?? Internet address is 12.1.1.2/24
?? MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
????? reliability 255/255, txload 1/255, rxload 1/255
? Encapsulation HDLC, crc 16, loopback not set
通过上面log可以看出，我们的猜测是正确的，可以下诊断；
故障排除：
通过上面log判断，链路两端接口封装类型不一致导致了故障，因此我们需要普及一下普通话，让它们封装一致了。就一R1的S1/1接口配置为准吧，现在我们将R2的接口封装类型也配置为PPP。
R2#configure terminal
Enter configuration commands, one per line.? End with CNTL/Z.
R2(config)#interface s1/1
R2(config-if)#encapsulation ppp
R2(config-if)#end
R2#
*May? 4 09:06:28.360: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up
*May? 4 09:06:28.821: %SYS-5-CONFIG_I: Configured from console by console
R2#
R1#
*May? 4 09:06:28.365: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up
R1#
现在两边都弹出了log，通知管理员接口的数据链路层up了。
R1#show ip interface brief s1/1
Interface????????????????? IP-Address????? OK??? Method?? Status??????????????? Protocol
Serial1/1????????????????? 12.1.1.1?????????? YES?? manual??? ? up?????????????????? up
R2#show ip interface brief s1/1
Interface????????????????? IP-Address????? OK??? Method??? Status??????????????? Protocol
Serial1/1????????????????? 12.1.1.2????????? YES???? manual???? up?????????????????? up?????
检查故障是否排除：
R1#ping 12.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/9 ms
R1#ping 12.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/17/18 ms
解惑：
通常情况下我们发现远端主机无法ping通，第一反应是需要测试本地主机接口卡工作是否正常，因此该同学在ping 12.1.1.2不通之后，又ping了12.1.1.1，证实了本地也不通。这他就疑惑了，对端不通你说因为接口协议down的，那咋自己也不通呢？
首先我们还原故障现象。
then
无论R1还是R2，当S1/1接口数据链路层down掉之后，我们来看下路由表：
R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
??????? D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
??????? N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
??????? E1 - OSPF external type 1, E2 - OSPF external type 2
??????? i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
??????? ia - IS-IS inter area, * - candidate default, U - per-user static route
??????? o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
??????? a - application route
??????? + - replicated route, % - next hop override
Gateway of last resort is not set
R1#
R2#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
??????? D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
??????? N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
??????? E1 - OSPF external type 1, E2 - OSPF external type 2
??????? i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
??????? ia - IS-IS inter area, * - candidate default, U - per-user static route
??????? o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
??????? a - application route
??????? + - replicated route, % - next hop override
Gateway of last resort is not set
R2#
路由表居然的空的！！！
接口的数据链路层都无法正常工作了，那么直连路由还有存在的必要么？数据也没办法完成链路层封装啊。一般的情况下，即使是发给自己的数据，路由器也需要有路由表项的嘛。没有表项，就好比一个人没有嘴巴，即使他知道别人给了他名字，心里也会默念，可就是说不出来呐。
原文地址：http://blog.51cto.com/dashu666/2112791
