“OSPF” DR与BDR

在OSPF的广播型网络和NBMA(非广播多路访问)类型网络中，如果网络中有n台路由器，若任意两台路由器之间都要建立邻接关系，则需要建立nx(n-1)/2个邻接关系，即当路由器很多时，则需要建立维护的邻接关系就很多，两两之间需要发送的报文也就很多，这会造成很多内容重复的报文在网络中传递，浪费了设备的带宽资源。因此在广播和NBMA类型网络中，OSPF协议定义了指定路由器DR（Designated Router），即所有其它路由器都只将各自的链路状态信息发送给DR，再由DR以组播方式发送至所有路由器，大大减少了OSPF数据包的发送。

如果DR由于某种故障而失效，此时网络中必须重新选举DR，并同步链路状态信息，这都需要较长的时间。为了缩短这个过程，OSPF协议定义了BDR（Backup Designated Router）的概念，作为DR备份路由器的备份，当DR路由器失效时，BDR成为DR，并再选择新的BDR路由器。其它非DR/BDR路由器都称为DR Other 路由器。

每一个含有至少两个路由器的广播类型网络或NBMA类型网络都会选举一个DR和BDR。选举规则是首先比较DR优先级，优先级高者成为DR，次高的成为BDR。如果一台路由器的DR优先级为0，则不参与选举。需要注意，DR是在某个广播域或者NBMA网段内进行选举的，是针对路由器的接口而言的。某台路由器在一个接口上可能是DR，，在另一个接口上有可能是BDR，或者是DR Router。

若DR，BDR已经选举完毕，人为修改任何一台路由器的DR优先级值为最大，也不会抢占成为新的DR,BDR，即OSPF的DR/BDR选举是非抢占的。

R1/R2/R3/R4 4台路由器分属于不同的部门，通过交换机S1互联。每台路由器运行OSPF路由协议，运行在区域0内，使公司各部门互通。路由器通过广播网络互联，OSPF会选举DR和BDR，现配置性能较好的R1成为DR，性能次之的R2成为BDR，而性能最差的R4不能参加DR和BDR的选举，由此来完成网络优化。 //接口配置，环回口配置 [R1]int g 0/0/0 [R1-GigabitEthernet0/0/0]ip add 172.16.1.1 24 [R1-GigabitEthernet0/0/0]int loopback 0 [R1-LoopBack0]ip add 1.1.1.1 32

[R2]int g 0/0/0 [R2-GigabitEthernet0/0/0]ip add 172.16.2.1 24 [R2-GigabitEthernet0/0/0]int loopback 0 [R2-LoopBack0]ip add 2.2.2.2 32

[R3]int g 0/0/0 [R3-GigabitEthernet0/0/0]ip add 172.16.3.1 24 [R3-GigabitEthernet0/0/0]int loopback 0 [R3-LoopBack0]ip add 3.3.3.3 32

[R4]int g 0/0/0 [R4-GigabitEthernet0/0/0]ip add 172.16.4.1 24 [R4-GigabitEthernet0/0/0]int loopback 0 [R4-LoopBack0]ip add 4.4.4.4 32

//4台路由器配置OSPF网络配置，各路由器的环回接口地址作为Router-ID，并运行在区域0内。 [R1]router id 1.1.1.1 [R1]ospf 1 [R1-ospf-1]area 0 [R1-ospf-1-area-0.0.0.0]network 172.16.1.0 0.0.0.255

reset ospf process //重启路由器上的OSPF进程 Warning: The OSPF process will be reset. Continue? [Y/N]:y

[R2]router id 2.2.2.2 [R2]ospf 1 [R2-ospf-1]area 0 [R2-ospf-1-area-0.0.0.0]network 172.16.1.0 0.0.0.255

reset ospf process Warning: The OSPF process will be reset. Continue? [Y/N]:y

[R3]router id 3.3.3.3 [R3]ospf 1 [R3-ospf-1]area 0 [R3-ospf-1-area-0.0.0.0]network 172.16.1.0 0.0.0.255

reset ospf process Warning: The OSPF process will be reset. Continue? [Y/N]:y

[R4]router id 4.4.4.4 [R4]ospf 1 [R4-ospf-1]area 0 [R4-ospf-1-area-0.0.0.0]network 172.16.1.0 0.0.0.255

reset ospf process Warning: The OSPF process will be reset. Continue? [Y/N]:y

//查看OSPF邻居建立情况 [R1]dis ospf peer brief

//查看OSPF网络中DR/BDR的选举情况 [R1]dis ospf peer

OSPF Process 1 with Router ID 1.1.1.1 Neighbors

Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighbors Router ID: 2.2.2.2 Address: 172.16.1.2 State: 2-Way Mode:Nbr is Master Priority: 1 DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 //可以看到R4为OSPF网络中的DR，R3为BDR，默认情况下每台路由器上的DR优先级为1 Dead timer due in 39 sec //此时是通过Router-ID的数值高低进行比较 Retrans timer interval: 0 Neighbor is up for 00:00:00 Authentication Sequence: [ 0 ]

Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1 DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 Dead timer due in 36 sec Retrans timer interval: 5 Neighbor is up for 00:00:04 Authentication Sequence: [ 0 ]

Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 1 DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 Dead timer due in 32 sec Retrans timer interval: 5 Neighbor is up for 00:00:18 Authentication Sequence: [ 0 ]

//在每台路由器接口下使用 “ospf network-type p2mp”修改OSPF网络类型为点到多点 其它设备不赘述 [R1]int g 0/0/0 [R1-GigabitEthernet0/0/0]ospf network-type p2mp

//观察选举情况 [R1]dis ospf peer

OSPF Process 1 with Router ID 1.1.1.1 Neighbors

Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighbors Router ID: 2.2.2.2 Address: 172.16.1.2 State: Init Mode:Nbr is Master Priority: 1 DR: None BDR: None MTU: 0 //DR/BDR都为None，验证了点到多点网络类型中不选举DR/BDR，点到点也是 Dead timer due in 112 sec Retrans timer interval: 0 Neighbor is up for 00:00:00 Authentication Sequence: [ 0 ]

Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1 DR: None BDR: None MTU: 0 Dead timer due in 96 sec Retrans timer interval: 5 Neighbor is up for 00:00:14 Authentication Sequence: [ 0 ]

Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 1 DR: None BDR: None MTU: 0 Dead timer due in 108 sec Retrans timer interval: 5 Neighbor is up for 00:00:14 Authentication Sequence: [ 0 ]

//还原网络类型为默认的广播网络类型 [R1]int g 0/0/0 [R1-GigabitEthernet0/0/0]ospf network-type broadcast //其它设备配置不赘述

//修改R1上GE0/0/0的接口优先级为100，R2为50，R4为0，R3保持默认不变。重启OSPF进程 reset ospf process [R1]int g 0/0/0 [R1-GigabitEthernet0/0/0]ospf dr-priority 100

[R2]int g 0/0/0 [R2-GigabitEthernet0/0/0]ospf dr-priority 50

[R4]int g 0/0/0 [R4-GigabitEthernet0/0/0]ospf dr-priority 0

//查看DR/BDR选举状态 [R1]dis ospf peer

OSPF Process 1 with Router ID 1.1.1.1 Neighbors

Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighbors Router ID: 2.2.2.2 Address: 172.16.1.2 State: Full Mode:Nbr is Master Priority: 50 DR: 172.16.1.1 BDR: 172.16.1.2 MTU: 0 //可以看到在广播网络中，R1为DR，R2为BDR。 Dead timer due in 39 sec Retrans timer interval: 5 Neighbor is up for 00:05:26 Authentication Sequence: [ 0 ]

Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1 DR: None BDR: None MTU: 0 Dead timer due in 32 sec Retrans timer interval: 5 Neighbor is up for 00:00:03 Authentication Sequence: [ 0 ]

Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 0 DR: 172.16.1.1 BDR: 172.16.1.2 MTU: 0 Dead timer due in 30 sec Retrans timer interval: 4 Neighbor is up for 00:07:04 Authentication Sequence: [ 0 ]