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　　(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 authenticatio