OSPF的DR和BDR

原理概述

原因

在OSPF的广播和NBMA类型网络中，如果有台路由器，若任意两台路由器之间都要建立邻接关系，则需要建立n*(n-1)/2个邻接关系，即当路由器很多时，则需要建立和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，这会造成内容重复的报文在网络中传递，浪费了设备的带宽资源。

定义：

DR（Designated Router，指定路由器），即所有其他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。
BDR（Backup Designated Router）的概念，作为DR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。其他非DR/BDR路由器都称为DR Other 路由器。

选举DR/BDR的规则：

DR优先级，如果DR优先级为0，则不参与选举
Router-ID

需要注意的是，DR是在某个广播或者NBMA网段内进行选举的，是针对路由器的接口而言的。某个路由器在一个接口上可能是DR，在另一个接口上可能是BDR，或者是DR Other。

实验内容

实验拓扑

实验步骤

1.配置路由器接口地址和环回接口地址
2.配置ospf网络

[R1]router id 1.1.1.1
[R1]ospf 1
[R1-ospf-1]area 0
[R1-ospf-1-area-0.0.0.0]net  172.16.1.0   0.0.0.255[R2]router id 2.2.2.2
[R2]ospf  1
[R2-ospf-1]area 0
[R2-ospf-1-area-0.0.0.0]net 172.16.1.0   0.0.0.255[R3]router id 3.3.3.3
[R3]ospf 1
[R3-ospf-1]area 0
[R3-ospf-1-area-0.0.0.0]net 172.16.1.0   0.0.0.255[R4]router id 4.4.4.4
[R4]ospf 1
[R4-ospf-1]area 0
[R4-ospf-1-area-0.0.0.0]net 172.16.1.0 0.0.0.255

配置完成后，同时重启4台路由器上的OSPF进程，或者直接同时重启路由器。

<R1>reset ospf process
<R2>reset ospf process
<R3>reset ospf process
<R4>reset ospf process

使用display ospf peer brief查看OSPF邻居建立情况

[R1]dis ospf peer brief OSPF Process 1 with Router ID 1.1.1.1Peer Statistic Information----------------------------------------------------------------------------Area Id          Interface                        Neighbor id      State    0.0.0.0          GigabitEthernet0/0/0             2.2.2.2          2-Way       0.0.0.0          GigabitEthernet0/0/0             3.3.3.3          Full        0.0.0.0          GigabitEthernet0/0/0             4.4.4.4          Full        ----------------------------------------------------------------------------可以观察到，R1已经和其他路由器成功建立起OSPF邻居关系。

查看DR/BDR状态

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2          Address: 172.16.1.2      State: 2-Way  Mode:Nbr is  Master  Priority: 1DR: 172.16.1.4  BDR: 172.16.1.3  MTU: 0    Dead timer due in 31  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: 1DR: 172.16.1.4  BDR: 172.16.1.3  MTU: 0    Dead timer due in 35  sec Retrans timer interval: 5 Neighbor is up for 00:05:13     Authentication Sequence: [ 0 ] Router ID: 4.4.4.4          Address: 172.16.1.4      State: Full  Mode:Nbr is  Master  Priority: 1DR: 172.16.1.4  BDR: 172.16.1.3  MTU: 0    Dead timer due in 36  sec Retrans timer interval: 0 Neighbor is up for 00:05:13     Authentication Sequence: [ 0 ] 可以观察到，R4为DR，R3为BDR。这是由于在默认情况下，每台路由器上的DR优先级都为1，此时是通过Router-ID的数值高低比较的。

3.验证网络类型点到多点的选举情况

[R1]int g0/0/0
[R1-GigabitEthernet0/0/0]ospf network-type p2mp[R2]int g0/0/0
[R2-GigabitEthernet0/0/0]ospf network-type p2mp[R3]int g0/0/0
[R3-GigabitEthernet0/0/0]ospf network-type p2mp[R4]int g0/0/0
[R4-GigabitEthernet0/0/0]ospf network-type p2mp

在R1上再次观察此时OSPF的选举情况

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2          Address: 172.16.1.2      State: Full  Mode:Nbr is  Master  Priority: 1DR: None   BDR: None   MTU: 0    Dead timer due in 112 sec Retrans timer interval: 5 Neighbor is up for 00:01:50     Authentication Sequence: [ 0 ] Router ID: 3.3.3.3          Address: 172.16.1.3      State: Full  Mode:Nbr is  Master  Priority: 1DR: None   BDR: None   MTU: 0    Dead timer due in 101 sec Retrans timer interval: 5 Neighbor is up for 00:00:47     Authentication Sequence: [ 0 ] Router ID: 4.4.4.4          Address: 172.16.1.4      State: Full  Mode:Nbr is  Master  Priority: 1DR: None   BDR: None   MTU: 0    Dead timer due in 103 sec Retrans timer interval: 0 Neighbor is up for 00:00:47     Authentication Sequence: [ 0 ] 可以观察到，DR/BDR都为None，验证了在点到多点的网络类型中不选举DR/BDR。在点到点的网络中也是。

4.根据网络需求影响DR/BDR选举
根据需求，需要使性能较好、处理能力较强的R1成为DR，性能次之的R2成为BDR，而性能最差的R4不能参加选举。

首先将OSPF网络类型还原为默认的广播网络类型

[R1]int g0/0/0
[R1-GigabitEthernet0/0/0]ospf network-type  broadcast[R2]int g0/0/0
[R2-GigabitEthernet0/0/0]ospf network-type  broadcast[R3]int g0/0/0
[R3-GigabitEthernet0/0/0]ospf network-type  broadcast[R4]int g0/0/0
[R4-GigabitEthernet0/0/0]ospf network-type  broadcast

配置完成后，修改R1上的接口的DR优先级为100，R2为50，R4为0，R3保持默认不变

[R1]int g0/0/0
[R1-GigabitEthernet0/0/0]ospf dr-priority 100[R2]int g0/0/0
[R2-GigabitEthernet0/0/0]ospf dr-priority 50[R4]int g0/0/0
[R4-GigabitEthernet0/0/0]ospf dr-priority 0

配置完成后，查看路由器的DR/BDR选举情况

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2          Address: 172.16.1.2      State: Full  Mode:Nbr is  Master  Priority: 50DR: 172.16.1.3  BDR: 172.16.1.1  MTU: 0    Dead timer due in 34  sec Retrans timer interval: 5 Neighbor is up for 00:01:08     Authentication Sequence: [ 0 ] Router ID: 3.3.3.3          Address: 172.16.1.3      State: Full  Mode:Nbr is  Master  Priority: 1DR: 172.16.1.3  BDR: 172.16.1.1  MTU: 0    Dead timer due in 34  sec Retrans timer interval: 0 Neighbor is up for 00:03:11     Authentication Sequence: [ 0 ] Router ID: 4.4.4.4          Address: 172.16.1.4      State: Full  Mode:Nbr is  Master  Priority: 0DR: 172.16.1.3  BDR: 172.16.1.1  MTU: 0    Dead timer due in 29  sec Retrans timer interval: 5 Neighbor is up for 00:01:03     Authentication Sequence: [ 0 ] 发现此时的DR与BDR都没有改变，即验证了OSPF的选举是非抢占性的。

同时重启4台路由器的OSPF进程，或者直接同时重启路由器。

[R1]dis ospf peerOSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2          Address: 172.16.1.2      State: Full  Mode:Nbr is  Master  Priority: 50DR: 172.16.1.1  BDR: 172.16.1.2  MTU: 0    Dead timer due in 31  sec Retrans timer interval: 4 Neighbor is up for 00:00:23     Authentication Sequence: [ 0 ] Router ID: 3.3.3.3          Address: 172.16.1.3      State: Full  Mode:Nbr is  Master  Priority: 1DR: 172.16.1.1  BDR: 172.16.1.2  MTU: 0    Dead timer due in 33  sec Retrans timer interval: 5 Neighbor is up for 00:00:23     Authentication Sequence: [ 0 ] Router ID: 4.4.4.4          Address: 172.16.1.4      State: Full  Mode:Nbr is  Master  Priority: 0DR: 172.16.1.1  BDR: 172.16.1.2  MTU: 0    Dead timer due in 33  sec Retrans timer interval: 5 Neighbor is up for 00:00:13     Authentication Sequence: [ 0 ] 此时在该广播网络中，R1为DR，R2为BDR。

实验目的

理解OSPF在哪种网络类型中会选举DR/BDR
掌握OSPF DR/BDR的选举规则
掌握如何更改设备接口上的DR优先级
理解OSPF DR/BDR选举的非抢占性特性