查阅资料后发现，有这么一句话，“在一个多址网络上，最先初始化启动的两台具有DR选取资格的路由器将成为DR和BDR路由器。”

这么说，可能是首先启动OSPF的路由器先把自己置为DR了，这样就并非最大RID的路由器是DR了。我们看看下面这个简单的实验。

如上拓扑配置好。R1的Loopback0口是1.1.1.1，R2的Loopback0口是2.2.2.2。

首先在R1上启用OSPF，R2上先不启用。
等待一小段时间后，再在R2上启用OSPF。

这时候查看R1的OSPF信息。

R1#show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.12.1/24, Area 0
Process ID 100, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 192.168.12.1
Backup Designated router (ID) 2.2.2.2, Interface address 192.168.12.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:05
Index 2/2, flood queue length 0
Next 0×0(0)/0×0(0)
Last flood scan length is 2, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2  (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Loopback0 is up, line protocol is up
Internet Address 1.1.1.1/24, Area 0
Process ID 100, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1
Loopback interface is treated as a stub Host

理论上说，R1和R2的RID分别是1.1.1.1和2.2.2.2，R2的RID应该是更大的，也就是说R2应该是DR才对，但是在这个网络中，首先启用的是R1，所以R1先把自己置为DR了，新加入的路由器，不管RID多大，也最多就是个BDR。
可以看一下R2上的OSPF端口信息。

R2#sh ip ospf int
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.12.2/24, Area 0
Process ID 100, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 192.168.12.1
Backup Designated router (ID) 2.2.2.2, Interface address 192.168.12.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:01
Index 2/2, flood queue length 0
Next 0×0(0)/0×0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 1.1.1.1  (Designated Router)
Suppress hello for 0 neighbor(s)
Loopback0 is up, line protocol is up
Internet Address 2.2.2.2/24, Area 0
Process ID 100, Router ID 2.2.2.2, Network Type LOOPBACK, Cost: 1
Loopback interface is treated as a stub Host

可以看到，R2就是个BDR。

用clear ip ospf process命令清楚ospf进程，然后重新选举DR和BDR，这次就正确了。

R1#show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.12.1/24, Area 0
Process ID 100, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 192.168.12.2
Backup Designated router (ID) 1.1.1.1, Interface address 192.168.12.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:08
Index 2/2, flood queue length 0
Next 0×0(0)/0×0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2  (Designated Router)
Suppress hello for 0 neighbor(s)

R2#show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.12.2/24, Area 0
Process ID 100, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 192.168.12.2
Backup Designated router (ID) 1.1.1.1, Interface address 192.168.12.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:09
Index 2/2, flood queue length 0
Next 0×0(0)/0×0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 1.1.1.1  (Backup Designated Router)
Suppress hello for 0 neighbor(s)

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文章标签: BDR, cisco, DR, DR选取, OSPF, RID, Router ID, 优先级, 路由器, 顺序初始化

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