1 概念
一个广播域选一个DR和一个BDR
10.1.1.4 priority最高，所以为DR。
10.1.1.3 priority次高，为BDR。
10.1.1.1，10.1.1.2为DR other。
如不手工配置，priority默认为1。
在4台路由器priority都相同的情况下，就要比较router id了。
如果优先级低的设备先做完配置，优先级高的设备后做配置，那优先级低的设备为DR。DR，BDR一旦确认就不会改变，除非设备宕机才会重新选举。
2修改优先级
[AR2]int g0/0/1
[AR2-GigabitEthernet0/0/1]ospf dr-priority 100
[AR2]quit
[AR2]dis ospf peer
OSPF Process 100 with Router ID 2.2.2.2
Neighbors
Area 0.0.0.0 interface 12.1.1.2(GigabitEthernet0/0/1)'s neighbors
Router ID: 1.1.1.1 Address: 12.1.1.1
State: Full Mode:Nbr is Slave Priority: 1
DR: 12.1.1.1 BDR: 12.1.1.2 MTU: 0
Dead timer due in 34 sec
Retrans timer interval: 5
Neighbor is up for 01:04:04
Authentication Sequence: [ 0 ]
Neighbors
Area 0.0.0.0 interface 23.1.1.2(GigabitEthernet0/0/0)'s neighbors
Router ID: 3.3.3.3 Address: 23.1.1.3
State: Full Mode:Nbr is Master Priority: 1
DR: 23.1.1.2 BDR: 23.1.1.3 MTU: 0
Dead timer due in 37 sec
Retrans timer interval: 5
Neighbor is up for 01:02:45
Authentication Sequence: [ 0 ]
3重置OSPF进程
[AR2]quit
<AR2>reset ospf 100 process
[AR1]quit
<AR1>reset ospf 100 process
AR2的端口12.1.1.2成为了DR，AR1的端口12.1.1.1为BDR
[AR2]dis ospf peer
OSPF Process 100 with Router ID 2.2.2.2
Neighbors
Area 0.0.0.0 interface 12.1.1.2(GigabitEthernet0/0/1)'s neighbors
Router ID: 1.1.1.1 Address: 12.1.1.1
State: Full Mode:Nbr is Slave Priority: 1
DR: 12.1.1.2 BDR: 12.1.1.1 MTU: 0
Dead timer due in 31 sec
Retrans timer interval: 5
Neighbor is up for 00:00:14
Authentication Sequence: [ 0 ]
Neighbors
Area 0.0.0.0 interface 23.1.1.2(GigabitEthernet0/0/0)'s neighbors
Router ID: 3.3.3.3 Address: 23.1.1.3
State: Full Mode:Nbr is Master Priority: 1
DR: 23.1.1.3 BDR: 23.1.1.2 MTU: 0
Dead timer due in 39 sec
Retrans timer interval: 4
Neighbor is up for 00:00:55
Authentication Sequence: [ 0 ]
4 cost值计算
<AR1>dis ospf routing
OSPF Process 100 with Router ID 1.1.1.1
Routing Tables
Routing for Network
Destination Cost Type NextHop AdvRouter Area
1.1.1.1/32 0 Stub 1.1.1.1 1.1.1.1 0.0.0.0
12.1.1.0/24 1 Transit 12.1.1.1 1.1.1.1 0.0.0.0
2.2.2.2/32 1 Stub 12.1.1.2 2.2.2.2 0.0.0.0
3.3.3.3/32 2 Stub 12.1.1.2 3.3.3.3 0.0.0.0
23.1.1.0/24 2 Transit 12.1.1.2 3.3.3.3 0.0.0.0
Total Nets: 5
Intra Area: 5 Inter Area: 0 ASE: 0 NSSA: 0
[AR1]dis ospf int g0/0/0
OSPF Process 100 with Router ID 1.1.1.1
Interfaces
Interface: 12.1.1.1 (GigabitEthernet0/0/0)
Cost: 1 State: BDR Type: Broadcast MTU: 1500
Priority: 1
Designated Router: 12.1.1.2
Backup Designated Router: 12.1.1.1
Timers: Hello 10 , Dead 40 , Poll 120 , Retransmit 5 , Transmit Delay 1
cost值计算为路由传递的方向，所有的cost值累加。
cost=1000000000/BW
bw为带宽，100000000为参考带宽
若带宽为100M=100*1000*1000=1000000000 所以100M带宽的cost值为1。1000M带宽的cost值也为1。这样就容易造成cost计算不准确。
所以建议调整参考带宽，默认为100Mbits/s。建议调整成10Gbits/s。
5调整参考带宽
[AR1]ospf 100
[AR1-ospf-100]bandwidth-reference ?
INTEGER<1-2147483648> The reference bandwidth (Mbits/s)
[AR1-ospf-100]bandwidth-reference 10000
[AR2]ospf 100
[AR2-ospf-100]bandwidth-reference 10000
[AR3]ospf 100
[AR3-ospf-100]bandwidth-reference 10000
参考带宽调整后，相应cost值也变化了
[AR1]dis ospf routing
OSPF Process 100 with Router ID 1.1.1.1
Routing Tables
Routing for Network
Destination Cost Type NextHop AdvRouter Area
1.1.1.1/32 0 Stub 1.1.1.1 1.1.1.1 0.0.0.0
12.1.1.0/24 10 Transit 12.1.1.1 1.1.1.1 0.0.0.0
2.2.2.2/32 10 Stub 12.1.1.2 2.2.2.2 0.0.0.0
3.3.3.3/32 20 Stub 12.1.1.2 3.3.3.3 0.0.0.0
23.1.1.0/24 20 Transit 12.1.1.2 3.3.3.3 0.0.0.0
Total Nets: 5
Intra Area: 5 Inter Area: 0 ASE: 0 NSSA: 0
