ospf邻接关系的建立
实验拓扑图：
路由器
R1
的
f0/0
连接路由器
R2
的
f0/0
接口。
具体的配置：
R1:
interface loopback 0
ip add 1.1.1 .1 255.0.0.0
interface f0/0
ip add 10.0.0 .1 255.0.0.0
no sh
router ospf 1
net 0.0.0 .0 0.0.0.0 a 0
R2:
interface loopback 0
ip add 2.2.2 .2 255.0.0.0
interface f0/0
ip add 10.0.0 .2 255.0.0.0
no sh
router ospf 1
net 0.0.0 .0 0.0.0.0 a 0
最基本的配置就完成了，为了观察
OSPF
邻接状态建立的过程，在路由器
R2
上执行
debug ip ospf adj
回显如下：
*Jul 31 13:59:17.919: OSPF: Cannot see ourself in hello from 1.1.1 .1 on FastEthernet0/0, state INIT
*Jul 31 13:59:17.923: OSPF: Neighbor change Event on interface FastEthernet0/0
*Jul 31 13:59:17.923: OSPF: DR/BDR election on FastEthernet0/0
*Jul 31 13:59:17.927: OSPF: Elect BDR 0.0.0 .0
*Jul 31 13:59:17.927: OSPF: Elect DR 2.2.2 .2
*Jul 31 13:59:17.927: DR: 2.2.2 .2 (Id) BDR: none
*Jul 31 13:59:18.423: OSPF: Build router LSA for area 0, router ID 2.2.2 .2, seq 0x8000000B
*Jul 31 13:59:18.423: OSPF: No full nbrs to build Net Lsa for interface FastEthernet0/0
*Jul 31 13:59:18.427: OSPF: Build network LSA for FastEthernet0/0, router ID 2.2.2 .2
*Jul 31 13:59:18.427: OSPF: Build network LSA for FastEthernet0/0, router ID 2.2.2 .2
*Jul 31 13:59:20.307: OSPF: Rcv DBD from 1.1.1 .1 on FastEthernet0/0 seq 0x1D2F opt 0x52 flag 0x7 len 32 mtu 1500 state INIT
*Jul 31 13:59:20.311: OSPF: 2 Way Communication to 1.1.1 .1 on FastEthernet0/0, state 2WAY
*Jul 31 13:59:20.311: OSPF: Neighbor change Event on interface FastEthernet0/0
*Jul 31 13:59:20.315: OSPF: DR/BDR election on FastEthernet0/0
*Jul 31 13:59:20.315: OSPF: Elect BDR 1.1.1 .1
*Jul 31 13:59:20.315: OSPF: Elect DR 2.2.2 .2
*Jul 31 13:59:20.319: DR: 2.2.2 .2 (Id) BDR: 1.1.1.1 (Id)
*Jul 31 13:59:20.319: OSPF: Send DBD to 1.1.1 .1 on FastEthernet0/0 seq 0xCEF opt 0x52 flag 0x7 len 32
*Jul 31 13:59:20.323: OSPF: First DBD and we are not SLAVE
*Jul 31 13:59:20.375: OSPF: Rcv DBD from 1.1.1 .1 on FastEthernet0/0 seq 0xCEF opt 0x52 flag 0x2 len 52 mtu 1500 state EXSTART
*Jul 31 13:59:20.379: OSPF: NBR Negotiation Done. We are the MASTER
*Jul 31 13:59:20.379: OSPF: Send DBD to 1.1.1 .1 on FastEthernet0/0 seq 0xCF0 opt 0x52 flag 0x3 len 52
*Jul 31 13:59:20.423: OSPF: Rcv DBD from 1.1.1 .1 on FastEthernet0/0 seq 0xCF0 opt 0x52 flag 0x0 len 32 mtu 1500 state EXCHANGE
*Jul 31 13:59:20.427: OSPF: Send DBD to 1.1.1 .1 on FastEthernet0/0 seq 0xCF1 opt 0x52 flag 0x1 len 32
*Jul 31 13:59:20.463: OSPF: Rcv DBD from 1.1.1 .1 on FastEthernet0/0 seq 0xCF1 opt 0x52 flag 0x0 len 32 mtu 1500 state EXCHANGE
*Jul 31 13:59:20.467: OSPF: Exchange Done with 1.1.1 .1 on FastEthernet0/0
*Jul 31 13:59:20.467: OSPF: Synchronized with 1.1.1 .1 on FastEthernet0/0, state FULL
*Jul 31 13:59:20.471: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1 .1 on FastEthernet0/0 from LOADING to FULL, Loading Done
OSPF
建立邻接关系需要经过
DOWN
，
INIT
，
TWO-WAY
，
EXSTART
，
EXCHANGE
，
FULL
，
LOAD DONE
这几个过程。如果一切正常是这样的。
如果把
R1
的
f0/0
接口的
MTU
改为
1000
，默认为
1500
，邻接关系在卡在
EXSTART
状态。
OSPF
邻居在建立邻接关系的过程中，在
EXSTART
这个阶段发送
DBD
包，对
MTU
的大小有一定的限制，可以在接口模式下配置：
ip ospf mtu-ignore,
忽略
MTU
对发送
DBD
包的影响。
R1
：
show ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2
.2 1 EXSTART/DR 00:00:38 10.0.0 .2 FastEthernet0/0
debug ip ospf adj
*Jul 31 14:16:10.343: OSPF: Send DBD to 2.2.2 .2 on FastEthernet0/0 seq 0x2648 opt 0x52 flag 0x7 len 32
*Jul 31 14:16:10.343: OSPF: Retransmitting DBD to 2.2.2 .2 on FastEthernet0/0 [8]
*Jul 31 14:16:10.395: OSPF: Rcv DBD from 2.2.2 .2 on FastEthernet0/0 seq 0x25C3 opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART
*Jul 31 14:16:10.395: OSPF: Nbr 2.2.2 .2 has larger interface MTU
R2:show ip ospf neigh
Neighbor ID Pri State Dead Time Address Interface
1.1.1
.1 1 EXSTART/BDR 00:00:37 10.0.0 .1 FastEthernet0/0
debug ip ospf adj
Jul 31 14:19:50.351: OSPF: Send DBD to 1.1.1 .1 on FastEthernet0/0 seq 0x23A4 opt 0x52 flag 0x7 len 32
*Jul 31 14:19:50.355: OSPF: Retransmitting DBD to 1.1.1 .1 on FastEthernet0/0 [12]
*Jul 31 14:19:55.323: OSPF: Rcv DBD from 1.1.1 .1 on FastEthernet0/0 seq 0xCF opt 0x52 flag 0x7 len 32 mtu 1000 state EXSTART
*Jul 31 14:19:55.327: OSPF: First DBD and we are not SLAVE
如果把
R1
和
R2
的优先级都改为
0
（
ip ospf priority 0
）的话，邻接状态就停在了
TWO-WAY
状态了。
R1:
interface f0/0
ip ospf priority 0
R2:
interface f0/0
ip ospf priority 0
R1: show ip ospf neigh
Neighbor ID Pri State Dead Time Address Interface
2.2.2
.2 0 2WAY/DROTHER 00:00:36 10.0.0 .2 FastEthernet0/
debug ip ospf adj
*Jul 31 14:25:30.319: OSPF: 2 Way Communication to 2.2.2 .2 on FastEthernet0/0, state 2WAY
*Jul 31 14:25:30.323: OSPF: Neighbor change Event on interface FastEthernet0/0
*Jul 31 14:25:30.323: OSPF: DR/BDR election on FastEthernet0/0
*Jul 31 14:25:30.327: OSPF: Elect BDR 0.0.0 .0
*Jul 31 14:25:30.327: OSPF: Elect DR 0.0.0 .0
*Jul 31 14:25:30.327: DR: none BDR: none
因为选举
DR
和
BDR
在广播网络中是必须的，且在
TWO-WAY
状态时就必须选举
DR
，没有确定的
DR
就不能进行
DBD
的发送，进而交换
LSA
更是不可能的。
同时如果两个路由器的接口不在同一区域，或是
HELLO
，
DEAD
时间间隔不一致，或是没有相同的末节区域标记，相同的认证密钥（如果有认证的话）都不能建立正常的邻接关系。
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