Cisco IOS OSPF邻居关系不能正常建立的原因和解决方法

R2#show ip ospf interface Ethernet 0
Ethernet0 is up, line protocol is up
Internet Address 131.108.1.2/24, Area 0
Process ID 1, Router ID 131.108.1.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 131.108.1.2, Interface address 131.108.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
No Hellos (Passive interface)
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)

R1#
interface Ethernet0
ip address 131.108.1.1 255.255.255.0
ip access-group 100 in
!
access-list 100 permit tcp any any
access-list 100 permit udp any any
access-list 101 permit ip 131.108.1.0 0.0.0.255 host 224.0.0.5
R2#
interface Ethernet0
ip address 131.108.1.2 255.255.255.0
ip access-group 100 in
!
access-list 100 permit tcp any any
access-list 100 permit udp any any
access-list 101 permit ip 131.108.1.0 0.0.0.255 host 224.0.0.5
R2#debug ip packet 101 detail
IP packet debugging is on (detailed) for access list 101
IP: s=131.108.1.2 (Ethernet0), d=224.0.0.5, len 68, access denied, proto=89
这时候的邻居关系是INIT
R2#show ip ospf neighbor
Neighbor ID   Pri  State    Dead Time   Address         Interface
131.108.2.1    1  INIT/-   00:00:33    131.108.1.1     Ethernet0
R1#show access-list 101
Extended IP access list 101
  permit ip 131.108.1.0 0.0.0.3 host 224.0.0.5 (8 matches)
R1#debug ip packet 101 detail
IP packet debugging is on (detailed) for access list 101
R1#
IP: s=131.108.1.1 (local), d=224.0.0.5 (Ethernet0), len 60, sending broad/multicast,
proto
=89
IP: s=131.108.1.2 (Ethernet0), d=224.0.0.5, len 82, access denied, proto=89
IP: s=131.108.1.1 (local), d=224.0.0.5 (Ethernet0), len 60, sending broad/multicast,
proto
=89
IP: s=131.108.1.2 (Ethernet0), d=224.0.0.5, len 82,access denied, proto=89

R1#debug ip ospf adj
OSPF adjacency events debugging is on
R1#
OSPF: Rcv pkt from 131.108.1.2, Ethernet0, area 0.0.0.0
    mismatch area 0.0.0.1 in the header
R2#show log
%OSPF-4-ERRRCV: Received invalid packet: mismatch area ID, from backbone area must be
      virtual-link but not found from 131.108.1.1, Ethernet0

R1#debug ip ospf adj
OSPF adjacency events debugging is on
R1#
OSPF: Rcv hello from 131.108.0.1 area 1 from Ethernet0 131.108.1.2
OSPF: Hello from 131.108.1.2 with mismatched Stub/Transit area option bit
R1#debug ip ospf adj
OSPF adjacency events debugging is on
R1#
OSPF: Rcv hello from 131.108.0.1 area 1 from Ethernet0 131.108.1.2
OSPF: Hello from 131.108.1.2 with mismatched NSSA option bit

R2#
interface FastEthernet0/0
ip address 131.108.1.2 255.255.255.0 secondary
ip address 131.108.4.2 255.255.255.0
R1#
interface Ethernet0
ip address 131.108.1.1 255.255.255.0
R2#debug ip ospf adj
OSPF adjacency events debugging is on
R2#
OSPF: Rcv pkt from 131.108.1.1, FastEthernet0/0, area 0.0.0.1 : src not on the same   network

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

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

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

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
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