Cisco IOS OSPF邻居关系不能正常建立原因和解决

(14)接口两边的MTU 不匹配

　　在cisco ios 12.0.3中加入了对mtu的检测，而低于这个版本的ios会忽略mtu检测，需要查看router 的ios version，debug ip ospf adj的结果是发现有大量的dbd重传，出现loading的原因就是一边的router需要对mtu进行检测，而一边忽略。

R2#debug ip ospf adj

OSPF adjacency events debugging is on

R2#

OSPF: Retransmitting request to 131.108.2.1 on Serial0

OSPF: Database request to 131.108.2.1

OSPF: sent LS REQ packet to 131.108.1.1, length 12

OSPF: Retransmitting request to 131.108.2.1 on Serial0

解决方法是接口命令：mtu 4470而不是：ip mtu 4470

这时候的状态是 loading 。

R2#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

131.108.2.1 1 LOADING/- 00:00:37 131.108.1.1 Serial0

　　还有一种mtu不匹配的情况是邻居关系是EXSTART，因为两边的mtu都需要检测，而mtu大小又不匹配.

R2#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

131.108.2.1 1 EXSTART/- 00:00:33 131.108.1.1 Serial0

R1#debug ip ospf adj

OSPF: Retransmitting DBD to 131.108.1.2 on Serial0.1

OSPF: Send DBD to 131.108.1.2 on Serial0.1 seq 0x1E55 opt 0x2 flag 0x7 len 32

OSPF: Rcv DBD from 131.108.1.2 on Serial0.1 seq 0x22AB opt 0x2 flag 0x7 len 32 mtu 1500

state EXSTART

OSPF: Nbr 131.108.1.2 has larger interface MTU

　　(15)在broadcast网络上需要选举dr，而这时候两边的优先级又都是0

　　(16)在Asynchronous Interfaces上的OSPF邻居关系

R1#

interface Async1

description ASYNC LINE TO R2

ip address 131.108.1.1 255.255.255.0

encapsulation ppp

async mode dedicated

dialer in-band

dialer map ip 131.108.1.2 name Router2 broadcast

dialer-group 1

ppp authentication chap

R2#

interface Async1

description ASYNC LINE TO R1

ip address 131.108.1.2 255.255.255.0

encapsulation ppp

async mode dedicated

dialer in-band

dialer map ip 131.108.1.1 name Router2 broadcast

dialer-group 1

ppp authentication chap

　　解决方法是在两个使用async default routing 或者asyn dynamic routing 接口命令

　　(15)两个路由器的Router id相同，ospf的master和slave关系不能确定

R2#debug ip ospf adj

OSPF: Retransmitting DBD to 131.108.2.1 on Serial0

OSPF: Send DBD to 131.108.2.1 on Serial0 seq 0x793 opt 0x2 flag 0x7 len 32

OSPF: Rcv DBD from 131.108.2.1 on Serial0 seq 0x25F7 opt 0x2 flag 0x7 len 32 mtu 0 state

EXSTART

OSPF: First DBD and we are not SLAVE

　　(16)因为router接口的物理问题而导致的 ospf packet checksum错误，接着就重传的问题。

R2#show log

%OSPF-4-ERRRCV: Received invalid packet: Bad Checksum from 131.108.1.1, Serial0

%OSPF-4-ERRRCV: Received invalid packet: Bad Checksum from 131.108.1.1, Serial0

R2#debug ip ospf adj

R2#

OSPF: Retransmitting request to 131.108.2.1 on Serial0

OSPF: Database request to 131.108.2.1