深入解析帧中继ping不通自身ip的原因

 

 作者：Komy-D      

       出处：http://hi.baidu.com/51cmdshell(转载注明出处)

首先在帧中继网云中配置pvc dlci和映射，这里不再赘述

R1配置如下：

!
interface Serial0/0/0
no ip address
encapsulation frame-relay
!
interface Serial0/0/0.102 point-to-point
ip address 1.1.1.1 255.255.255.0
frame-relay interface-dlci 102
!
interface Serial0/0/0.103 point-to-point
ip address 2.2.2.1 255.255.255.0
frame-relay interface-dlci 103
!

R2配置如下：

!
interface Serial0/0/0
ip address 1.1.1.2 255.255.255.0
encapsulation frame-relay
!

R3配置如下：

!
interface Serial0/0/0
ip address 2.2.2.2 255.255.255.0
encapsulation frame-relay
!

R1、R2、R3的s0/0/0接口均为启用状态

配置完毕后，R1与R2、R3能ping通，R2和R3不能ping通。但问题随之而来，R1能ping通自己的两个子接口地址1.1.1.1和2.2.2.1

R1#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 100 percent (5/5), round-trip min/avg/max = 13/16/25 ms

R1#ping 2.2.2.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/13/17 ms

R2无法ping通自身的物理接口IP地址：

R2#ping 1.1.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

R3无法ping通自身的物理接口IP地址：

R3#ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

这是为什么呢？自身子接口的IP可以ping通，物理接口却不行，在我看来，配置R1两个子接口时使用的frame-relay interface-dlci DLCI命令，就把DLCI和子接口关联了起来，形成了子接口IP与DLCI的映射关系。

        由于物理接口（R2、R3的配置）默认使用inverse-arp，路由器就会与网云中的帧中继交换机进行lmi通信，获得dlci信息，然后发送inarp包获取目标的IP地址，形成动态映射。这种方式只是与对端的路由进行inarp，而忽略了其本身。

         帧中继中通讯是基于DLCI的，它相当于以太网中的物理地址MAC，物理接口中没有自动创建本地接口与DLCI的映射，也就无法通信。子接口配置中由于用的frame-relay interface-dlci DLCI命令，把DLCI和子接口关联了起来，形成了子接口IP与DLCI的映射关系。

这些均为本人的个人见解，不足之处请提出！谢谢

现在在R2、R3中的s0/0/0的接口模式下分别执行

frame-relay map ip 1.1.1.2 201

frame-relay map ip 2.2.2.2 301

创建dlci与本地IP的映射关系，物理接口的本地IP成功ping通：

R2# ping 1.1.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/14/18 ms

R3# ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/14/18 ms

本文意义不大，算是研究下技术罢了，没有多大的实用价值

但看完本文后你会深入了解到为什么帧中继中ping不通自身IP的疑惑

PS：在wan的其他封装方式中，如ppp、hdlc均可ping通自身ip

关于此问题在ccna中的考题：

When troubleshooting a Frame Relay connection, what is the first step when performing a loopback
test?
A. Set the encapsulation of the interface to HDLC.
B. Place the CSU/DSU in local-loop mode.
C. Enable local-loop mode on the DCE Frame Relay router.
D. Verify that the encapsulation is set to Frame Relay.
Answer: A

 

补充：如果想让R2和R3能通互想ping通，可添加如下命令：

R2：

R2(config)#frame-relay map ip 2.2.2.1 201          //添加指向R1的103子接口的arp映射关系。

R2(config)#ip route 2.2.2.0 255.255.255.0 1.1.1.1  //添加指向2.2.2.0网段的路由。

R2(config)#do sh frame-relay map

Serial1/0 (up): ip 2.2.2.1 dlci 201(0xC9,0x3090), static,

              CISCO, status defined, active

Serial1/0 (up): ip 1.1.1.2 dlci 201(0xC9,0x3090), static,

              CISCO, status defined, active

Serial1/0 (up): ip 1.1.1.1 dlci 201(0xC9,0x3090), dynamic,

              broadcast,, status defined, active

 

R3（方法同上）：

R3(config)#frame-relay map ip 1.1.1.1 301

R3(config)#ip route 1.1.1.0 255.255.255.0 2.2.2.1

R3(config)#do sh frame-relay map

Serial1/0 (up): ip 1.1.1.1 dlci 301(0x12D,0x48D0), static,

              CISCO, status defined, active

Serial1/0 (up): ip 2.2.2.2 dlci 301(0x12D,0x48D0), static,

              CISCO, status defined, active

Serial1/0 (up): ip 2.2.2.1 dlci 301(0x12D,0x48D0), dynamic,

              broadcast,, status defined, active