R1、R2、R3上的预配置R1(config)#int f0/0R1(config-if)#ip ad 123.1.1.1 255.255.255.0R1(config-if)#no shR1(con
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R1、R2、R3上的预配置
R1(config)#int f0/0
R1(config-if)#ip ad 123.1.1.1 255.255.255.0
R1(config-if)#no sh
R1(config)#router os 11
R1(config-router)#net 123.1.1.1 0.0.0.255 area 0
R1(config-router)#end
2(config)#int f0/0
R2(config-if)#ip ad 123.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config)#router os 11
R2(config-router)#net 123.1.1.2 0.0.0.255 a 0
R2(config-router)#end
R3(config)#int f0/0
R3(config-if)#ip add 123.1.1.3 255.255.255.0
R3(config-if)#no sh
R3(config)#router os 11
R3(config-router)#net 123.1.1.3 0.0.0.255 area 0
R3(config-router)#end
oob-resync timeout 40
Hello due in 00:00:09
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 2, Adjacent neighbor count is 2
Adjacent with neighbor 123.1.1.2 (Backup Designated Router)
Adjacent with neighbor 123.1.1.3
Suppress hello for 0 neighbor(s)
R1#
R1#sh ip os nei
//在这里我们看到了两个邻居
Neighbor ID Pri State Dead Time Address Interface
123.1.1.2 1 FULL/BDR 00:00:35 123.1.1.2 FastEthernet0/0
123.1.1.3 1 FULL/DROTHER 00:00:39 123.1.1.3 FastEthernet0/0
R1#sh ip ospf nei detail
//查看ospf邻居详细情况
Neighbor 123.1.1.2, interface address 123.1.1.2
//邻居123.1.1.2的信息，下面我们可以看到123.1.1.3的信息
In the area 0 via interface FastEthernet0/0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 123.1.1.1 BDR is 123.1.1.2
//这里可以看到DR和BDR
Options is 0x52
LLS Options is 0x1 (LR)
Dead timer due in 00:00:39 //死亡时间的剩余里
Neighbor is up for 00:11:14 //与邻居建立关系时长
Index 1/1, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
Neighbor 123.1.1.3, interface address 123.1.1.3
//邻居123.1.1.2的信息，上面我们可以看到123.1.1.2的信息
In the area 0 via interface FastEthernet0/0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 123.1.1.1 BDR is 123.1.1.2
Options is 0x52
LLS Options is 0x1 (LR)
Dead timer due in 00:00:33
Neighbor is up for 00:09:44
Index 2/2, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
R1#
第二步：分析为什么R1为DR而ip较大的R3不是呢？
这里因为优先级相同的情况下，我们先配置的是R1.
第三步：通过清除邻居关系来影响DR和BDR的选举（三个都要进行）
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: y
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
123.1.1.2 1 FULL/BDR 00:00:34 123.1.1.2 FastEthernet0/0
123.1.1.3 1 FULL/DR 00:00:34 123.1.1.3 FastEthernet0/0
//在这里看到了R3为DR，R2为BDR，这复合了在优先级相同且没有回环接口的情况下ip最大的路由器为 DR说法
简单总结：
优先级 相同 先配置的为DR，routerID大为DR
优先级 不同 优先级大的为DR。
请原谅我在文章中多次使用图片，本人在学习时大量文档由word保存，是可以直接粘贴过来 的，但是每一次总有人从本站大量复制作品，迫不得已！！！
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