通过本实验可以了解：
1、 路由器OSPF的启动过程；
2、 启用路由接口，并通告网络所在路由；
3、 DR选举的控制；
4、 广播多路访问链路的特征；
实验拓扑图：
实验步骤：
配置各个路由器fa0/0和loopback 0的IP地址，并配置OSPF路由协议
R1的ospf配置如下：
R1(config)#router ospf 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 192.168.1.0 255.255.255.0 area 0
R1(config-router)#network 1.1.1.0 255.255.255.0 area 0
R2、R3、R4与之类似。
当所有配置完成之后，在R1、R2上查看路由信息，可以看到DR，BDR的选举正在进行
再到R4上执行“show ip ospf neighbor”
以上实验输出表明在该广播多路访问网络中，R4是DR，R3是BDR，R1和R2是DROTHER。
【注】：
1、为了避免路由器之间建立完全邻接关系而引起的大量开销，OSPF协议要求在多路访问链路中选举一个DR，每个路由器都与之建立邻接关系。选举DR的同时，也选举一个BDR。在DR失效的时候，BDR担负起DR的责任，而且所有其他路由器只与DR和BDR建立邻接关系；
2、DR和BDR有他们自己的组播地址224.0.0.6；
3、DR和BDR的选举是以各个网络为基础的，也就是说DR和BDR选举是一个；路由器的接口特性，而不是整个路由器的特性；
4、DR的选举规则为：
①首要因素是时间，最先启动的路由器被选举为DR；
②如果是同时启动，或者重新选举，则看接口优先级（范围是0--255），优先级最高的被选举为DR，默认情况下，多路访问网络的接口优先级为1，点到点的接口优先级为0，修改优先级的命令为“ip ospf priority ”，如果接口的优先级被置为0，那么该接口将不参与DR的选举；
③如果前两者相同，最后看路由器ID，路由器最高的被选举为DR；
5、DR选举是非抢占的，除非人为地重新选举，重新选举的方法有两种，一是路由器重启，二是执行“clear ip ospf process”命令；
在R1上执行show ip ospf int fastEthernet 0/0命令
R1#show ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
//网络类型为BROADCAST；
Transmit Delay is 1 sec, State DROTHER, Priority 1
//自己的State是DROTHER；
Designated Router (ID) 4.4.4.4, Interface address 192.168.1.4
//DR的路由器ID和接口地址；
Backup Designated router (ID) 3.3.3.3, Interface address 192.168.1.3
//BDR的路由器ID和接口地址；
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:09
Supports Link-local Signaling (LLS)
Index 2/2, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 3, Adjacent neighbor count is 2
//有3个邻居，只与R3、R4形成邻接关系，与R2只是邻居关系；
Adjacent with neighbor 3.3.3.3 (Backup Designated Router)
Adjacent with neighbor 4.4.4.4 (Designated Router)
//与R3、R4形成邻接关系；
Suppress hello for 0 neighbor(s)
在R4上执行show ip ospf int fastEthernet 0/0命令
R4#show ip ospf int fastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.4/24, Area 0
Process ID 1, Router ID 4.4.4.4, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
//自己是DR；
Designated Router (ID) 4.4.4.4, Interface address 192.168.1.4
//DR路由器的ID和接口地址；
Backup Designated router (ID) 3.3.3.3, Interface address 192.168.1.3
//BDR路由器的ID和接口地址；
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:07
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 4 msec, maximum is 4 msec
Neighbor Count is 3, Adjacent neighbor count is 3
//R4是DR，有3个邻居，并且全部形成邻接关系；
Adjacent with neighbor 1.1.1.1
Adjacent with neighbor 2.2.2.2
Adjacent with neighbor 3.3.3.3 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
从路由器的R1和R4的输出来看，邻居关系和邻接关系是不相同的，邻居关系是指达到2WAY状态的路由器，而邻接关系是指达到FULL状态的路由器。
Debug ip ospf adj 命令显示邻接关系的创建或中断的过程。在OSPF邻接关系创建的过程中，接口状态变化包括：DOWN，2WAY，EXSTART，EXCHANGE，LOADING，FULL
R4# debug ip ospf adj
OSPF adjacency events debugging is on
R4#clear ip ospf process
………………
*Mar 1 00:09:44.811: OSPF: Elect DR 3.3.3.3
*Mar 1 00:09:44.811: DR: 3.3.3.3 (Id) BDR: 4.4.4.4 (Id)
*Mar 1 00:09:47.495: OSPF: 3.3.3.3 address 192.168.1.3 on FastEthernet0/0 is dead
*Mar 1 00:09:47.495: OSPF: 3.3.3.3 address 192.168.1.3 on FastEthernet0/0 is dead, state DOWN
………………
*Mar 1 00:09:47.495: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired
*Mar 1 00:09:47.499: OSPF: Neighbor change Event on interface FastEthernet0/0
*Mar 1 00:09:47.499: OSPF: DR/BDR election on FastEthernet0/0
………………
*Mar 1 00:10:05.791: OSPF: 2 Way Communication to 3.3.3.3 on FastEthernet0/0, state 2WAY
*Mar 1 00:10:05.791: OSPF: Neighbor change Event on interface FastEthernet0/0
*Mar 1 00:10:05.791: OSPF: DR/BDR election on FastEthernet0/0
*Mar 1 00:10:05.795: OSPF: Elect BDR 1.1.1.1
*Mar 1 00:10:05.795: OSPF: Elect DR 4.4.4.4
*Mar 1 00:10:05.795: DR: 4.4.4.4 (Id) BDR: 1.1.1.1 (Id)
*Mar 1 00:10:05.795: OSPF: Send DBD to 3.3.3.3 on FastEthernet0/0 seq 0x6CD opt 0x52 flag 0x7 len 32
………………
*Mar 1 00:10:06.703: OSPF: Rcv DBD from 3.3.3.3 on FastEthernet0/0 seq 0x8AF opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART
*Mar 1 00:10:06.703: OSPF: First DBD and we are not SLAVE
*Mar 1 00:10:06.727: OSPF: Rcv DBD from 3.3.3.3 on FastEthernet0/0 seq 0x6CD opt 0x52 flag 0x2 len 132 mtu 1500 state EXSTART
*Mar 1 00:10:06.727: OSPF: NBR Negotiation Done. We are the MASTER
*Mar 1 00:10:06.731: OSPF: Send DBD to 3.3.3.3 on FastEthernet0/0 seq 0x6CE opt 0x52 flag 0x3 len 132
*Mar 1 00:10:07.063: OSPF: Rcv DBD from 3.3.3.3 on FastEthernet0/0 seq 0x6CE opt 0x52 flag 0x0 len 32 mtu 1500 state EXCHANGE
*Mar 1 00:10:07.063: OSPF: Send DBD to 3.3.3.3 on FastEthernet0/0 seq 0x6CF opt 0x52 flag 0x1 len 32
*Mar 1 00:10:07.111: OSPF: Rcv LS UPD from 3.3.3.3 on FastEthernet0/0 length 88 LSA count 1
*Mar 1 00:10:07.115: OSPF: Rcv LS UPD from 3.3.3.3 on FastEthernet0/0 length 68 LSA count 1
*Mar 1 00:10:07.183: OSPF: Rcv LS REQ from 3.3.3.3 on FastEthernet0/0 length 36 LSA count 1
*Mar 1 00:10:07.187: OSPF: Send UPD to 192.168.1.3 on FastEthernet0/0 length 52 LSA count 1
*Mar 1 00:10:07.187: OSPF: Rcv DBD from 3.3.3.3 on FastEthernet0/0 seq 0x6CF opt 0x52 flag 0x0 len 32 mtu 1500 state EXCHANGE
*Mar 1 00:10:07.187: OSPF: Exchange Done with 3.3.3.3 on FastEthernet0/0
*Mar 1 00:10:07.187: OSPF: Synchronized with 3.3.3.3 on FastEthernet0/0, state FULL
*Mar 1 00:10:07.191: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/0 from LOADING to FULL, Loading Done
