HCNA Routing&Switching之动态路由协议OSPF DR和BDR
前文我们了解了OSPF建立邻居关系的条件，回顾请参考https://www.cnblogs.com/qiuhom-1874/p/15032907.html；今天我们来聊一聊OSPF中的DR和BDR；
在说DR和BDR之前我们先来了解下OSPF的网络类型；
首先OSPF的网络类型是基于接口来划分，其次OSPF的网络类型是通过接口的二层封装报文的方式进行了划分，默认情况下OSPF把接口二层是以太网封装的网络称为广播类型，把接口二层是PPP或者HDLC封装的网络称为点到点类型；
在生活中，广播就是一个人说话，在同一个房间里的其他人都可以听到；在网络里，广播和这个类似；一个网络设备在一个广播域发送一条广播报文，在同一个广播域的所有设备都能收到该设备的广播；这样一来我们要想和其中的一台设备通讯怎么通讯呢？在二层为以太网封装的广播类型网络里，它是通过二层以太网封装目标MAC和源MAC；其原因是ip地址是变化的，如果单纯的使用ip地址不能够准确的标识某一台设备（因为ip地址是变化的），而MAC地址是设备的硬件地址，这个地址是设备出厂时就固定了，况且不会发生变化；所以确定一台设备的唯一性，我们可以通过MAC地址；有了通信双方的mac地址再结合三层的ip地址，我们就可以准确的将对应数据包发送给对方；
对于点到点类型的网络，其二层就不需要封装MAC地址，一条链路上就只有两个端点，在A端点发送数据B端点就一定能收到；
提示：广播类型的接口是以太网类型接口（就是我们常常看到的网口），而点到点类型的接口是串口；
在点对点类型的网络中，基于接口封装的不同又可以分非广播多路访问和点到多点，如下图
提示：默认情况下，OSPF认为帧中继（FR）、异步传输（ATM）的网络类型为NBMA；在帧中继网络里非广播多路访问和以太网中的广播类型，两者有一个相同的特点，就是都可以接多个设备；不同点是以太网二层封装需要封装mac地址，支持广播发送，而帧中继网络二层不需要封装mac地址，其原因是它不支持广播；不支持广播就意味着在ospf里不支持组播；不支持组播也就是说帧中继网络里，ospf只能通过单播的方式建立邻居；
实验：如下图两个拓扑，R1和R2为以太网类型接口，R3和R4为串口接口，在两个实验环境中配置ospf，抓包看看两者发送到hello包有什么不同
配置R1
sys sys R1 int g0/0/0 ip add 12.0.0.1 24 ospf 1 router-id 1.1.1.1 area 0 net 12.0.0.1 0.0.0.0 dis ip int b
配置R2
sys sys R2 int g0/0/0 ip add 12.0.0.2 24 ospf 1 router-id 2.2.2.2 area 0 net 12.0.0.2 0.0.0.0 dis ip int b
配置R3
sys sys R3 int s4/0/0 ip add 34.0.0.3 24 ospf 1 router-id 3.3.3.3 area 0 net 34.0.0.3 0.0.0.0 dis ip int b
配置R4
sys sys R4 int s4/0/0 ip add 34.0.0.4 24 ospf 1 router-id 4.4.4.4 area 0 net 34.0.0.4 0.0.0.0 dis ip int b
在R1或R2上抓包
在R3或R4上抓包
提示：默认情况串口链路的二层封装为ppp类型，这里抓包就选择ppp链路类型即可；
提示：从上面的抓包情况可以看到，在以太网封类型的网络环境下抓包，其二层封装了目标mac和源mac，而在点到点类型的网络环境下抓包，其二层就不是使用以太网协议而是用ppp协议，并且数据包里面也没有封装mac地址；
验证：查看对应ospf网络类型
在R1或R2上验证相关接口的网络类型
在R3或R4上验证相关接口的网络类型
更改R3的s4/0/0的链路封装为FR，看看对应的接口类型是什么
提示：可以看到修改了R3的s4/0/0接口的封装为FR以后，对应ospf就down掉了；并且对应接口的类型就变为了NBMA（非广播多路访问）；
OSPF DR和BDR
DR和BDR的作用
在上述的网络类型中，在多路访问网络类型中跑OSPF，每个路由器都会和其他路由器建立邻居，一个路由器发送一条链路更新，其他路由器都会跟着确认，这样一来，每个路由器都要经过两两的交互和确认，在一定程度上会带来路由器的性能的开销（hello包，DD，LSR，LSU，LSACK这些包过多，增加路由器的处理压力，从而导致路由器的性能降低）；为了降低数据包的交换，在OSPF中会选举DR和BDR；如下图
提示：广播型网络BMA和NBMA网络中，为了减少邻接关系的数量，从而减少数据包交换，次数，最终节省带宽，降低对路由器处理能力的压力，选举DR和BDR。
相关术语概述
DR：Designed Router，指定路由器，类似班长、总经理；
BDR：Backup DR，备用DR，类似副班长、副总经理；
DRouthers：类似普通学生、普通员工；
三者的关系：DR、BDR、DRothers之间都保持邻接关系（Full），DRothers之间保持邻居关系（Two-Way）；
通信地址：224.0.0.6向DR和BDR发送链路状态更新，224.0.0.5向所有OSPF路由器发送；
DR选举规则
1、首先比较Hello报文中携带的优先级，优先级范围是0-255，默认情况下都是1；优先级最高的被选举为DR，优先级次高的被选举为BDR；优先级为0的不参与选举；
2、如果优先级相同的情况下，比较router id大小，router id越大越优先；
3、选举不具有抢占性，除非当DR和BDR都失效或重启OSPF进程；
提示：DRothers的LSU向DR或BDR通过224.0.0.6发送，然后DR收到DRothers发送到LSU以后，会将对应的LSU通过224.0.0.5向其他路由器发送，其他路由器收到DR发送LSU以后，BDR向224.0.0.5直接确认，而DBothers则通过224.0.0.6向DR确认；总结一句话就是从DR或BDR发出的数据包都是发往224.0.0.5，DR和BDR都会监听224.0.0.6这个地址；DRothers发出的数据会通过224.0.0.6向DR或BDR发送；
实验：如下图，全网运行ospf，看看对应的数据包会这么发送，对应路由器之间是什么关系
配置R1
sys sys R1 int g0/0/0 ip add 1.0.0.1 24 ospf 1 router-id 1.1.1.1 a 0 net 1.0.0.0 0.0.0.255 dis ip int b dis ospf peer brief
在R1上查看g0/0/0的接口信息，看看谁是DR和BDR
提示：可以看到只配置了R1上的OSPF以后，对应DR就是它自己，因为此时没有其他设备和它竞选DR，所以只有它是DR；对于BDR来说，它不知道，因为目前就只有它自己在ospf里面，没有看到其他设备；
配置R2
sys sys R2 int g0/0/0 ip add 1.0.0.2 24 ospf 1 router-id 2.2.2.2 a 0 net 1.0.0.0 0.0.0.255 dis ip int b dis ospf peer brief
<Huawei>sys
Enter system view, return user view with Ctrl+Z.
[Huawei]sys R2
[R2]int g0/0/0
[R2-GigabitEthernet0/0/0]ip add 1.0.0.2 24
[R2-GigabitEthernet0/0/0]ospf 1 rout
Jul 24 2021 12:08:39-08:00 R2 %%01IFNET/4/LINK_STATE(l)[0]:The line protocol IP on the interface GigabitEthernet0/0/0 has entered the UP state.
[R2-GigabitEthernet0/0/0]ospf 1 router-id 2.2.2.2
[R2-ospf-1]a 0
[R2-ospf-1-area-0.0.0.0]net 1.0.0.0 0.0.0.255
[R2-ospf-1-area-0.0.0.0]dis ip int b
*down: administratively down
^down: standby
(l): loopback
(s): spoofing
The number of interface that is UP in Physical is 2
The number of interface that is DOWN in Physical is 2
The number of interface that is UP in Protocol is 2
The number of interface that is DOWN in Protocol is 2
Interface IP Address/Mask Physical Protocol
GigabitEthernet0/0/0 1.0.0.2/24 up up
GigabitEthernet0/0/1 unassigned down down
GigabitEthernet0/0/2 unassigned down down
NULL0 unassigned up up(s)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[1]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[2]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[3]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=ExStart)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[4]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=NegotiationDone, NeighborPreviousState=ExStart, NeighborCurrentState=Exchange)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[5]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=ExchangeDone, NeighborPreviousState=Exchange, NeighborCurrentState=Loading)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:08:44-08:00 R2 %%01OSPF/4/NBR_CHANGE_E(l)[6]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=LoadingDone, NeighborPreviousState=Loading, NeighborCurrentState=Full)
[R2-ospf-1-area-0.0.0.0]dis ospf peer brief
OSPF Process 1 with Router ID 2.2.2.2
Peer Statistic Information
----------------------------------------------------------------------------
Area Id Interface Neighbor id State
0.0.0.0 GigabitEthernet0/0/0 1.1.1.1 Full
----------------------------------------------------------------------------
[R2-ospf-1-area-0.0.0.0]
提示：可以看到当R2配置好ospf以后，它会和R1先建立其邻接关系；
再次在R1上查看g0/0/0的接口信息，看看此时谁是DR和BDR
提示：此时可以看到DR是R1的接口ip地址，BDR是R2的接口地址；
配置R3
sys sys R3 int g0/0/0 ip add 1.0.0.3 24 ospf 1 router-id 3.3.3.3 a 0 net 1.0.0.0 0.0.0.255 dis ip int b dis ospf peer brief
<Huawei>sys
Enter system view, return user view with Ctrl+Z.
[Huawei]sys R3
[R3]int g0/0/0
[R3-GigabitEthernet0/0/0]ip add 1.0.0.3 24
Jul 24 2021 12:14:41-08:00 R3 %%01IFNET/4/LINK_STATE(l)[0]:The line protocol IP on the interface GigabitEthernet0/0/0 has entered the UP state.
[R3-GigabitEthernet0/0/0]ospf 1 router-id 3.3.3.3
[R3-ospf-1]a 0
[R3-ospf-1-area-0.0.0.0]net 1.0.0.0 0.0.0.255
[R3-ospf-1-area-0.0.0.0]dis ip int b
*down: administratively down
^down: standby
(l): loopback
(s): spoofing
The number of interface that is UP in Physical is 2
The number of interface that is DOWN in Physical is 2
The number of interface that is UP in Protocol is 2
The number of interface that is DOWN in Protocol is 2
Interface IP Address/Mask Physical Protocol
GigabitEthernet0/0/0 1.0.0.3/24 up up
GigabitEthernet0/0/1 unassigned down down
GigabitEthernet0/0/2 unassigned down down
NULL0 unassigned up up(s)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:45-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[1]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:45-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[2]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[3]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[4]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[5]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=ExStart)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[6]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=ExStart)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[7]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=NegotiationDone, NeighborPreviousState=ExStart, NeighborCurrentState=Exchange)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[8]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=NegotiationDone, NeighborPreviousState=ExStart, NeighborCurrentState=Exchange)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[9]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=ExchangeDone, NeighborPreviousState=Exchange, NeighborCurrentState=Loading)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[10]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=LoadingDone, NeighborPreviousState=Loading, NeighborCurrentState=Full)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[11]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=ExchangeDone, NeighborPreviousState=Exchange, NeighborCurrentState=Loading)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:14:46-08:00 R3 %%01OSPF/4/NBR_CHANGE_E(l)[12]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=LoadingDone, NeighborPreviousState=Loading, NeighborCurrentState=Full)
[R3-ospf-1-area-0.0.0.0]dis ospf peer brief
OSPF Process 1 with Router ID 3.3.3.3
Peer Statistic Information
----------------------------------------------------------------------------
Area Id Interface Neighbor id State
0.0.0.0 GigabitEthernet0/0/0 1.1.1.1 Full
0.0.0.0 GigabitEthernet0/0/0 2.2.2.2 Full
----------------------------------------------------------------------------
[R3-ospf-1-area-0.0.0.0]
提示：可以看到当R3配置好以后，对应R3和R1和R2都建立起邻接关系；
查看R3g0/0/0接口信息，看看DR和BDR变化没有？
提示：之所以DR和BDR没有发生变化，其原因是ospf已经选好了DR和BDR，而DR和BDR的选举是非抢占性，一旦选好，就不变了，除非重启ospf进程（所有路由器）来触发DR和BDR的重新选举；
配置R4
sys sys R4 int g0/0/0 ip add 1.0.0.4 24 ospf 1 router-id 4.4.4.4 a 0 net 1.0.0.0 0.0.0.255 dis ip int b dis ospf peer brief
<Huawei>sys
Enter system view, return user view with Ctrl+Z.
[Huawei]sys R4
[R4]int g0/0/0
[R4-GigabitEthernet0/0/0]ip add 1.0.0.4 24
Jul 24 2021 12:20:57-08:00 R4 %%01IFNET/4/LINK_STATE(l)[0]:The line protocol IP on the interface GigabitEthernet0/0/0 has entered the UP state.
[R4-GigabitEthernet0/0/0]ospf 1 router-id 4.4.4.4
[R4-ospf-1]a 0
[R4-ospf-1-area-0.0.0.0]net 1.0.0.0 0.0.0.255
[R4-ospf-1-area-0.0.0.0]dis ip int b
*down: administratively down
^down: standby
(l): loopback
(s): spoofing
The number of interface that is UP in Physical is 2
The number of interface that is DOWN in Physical is 2
The number of interface that is UP in Protocol is 2
The number of interface that is DOWN in Protocol is 2
Interface IP Address/Mask Physical Protocol
GigabitEthernet0/0/0 1.0.0.4/24 up up
GigabitEthernet0/0/1 unassigned down down
GigabitEthernet0/0/2 unassigned down down
NULL0 unassigned up up(s)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[1]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[2]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[3]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=ExStart)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[4]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=NegotiationDone, NeighborPreviousState=ExStart, NeighborCurrentState=Exchange)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[5]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=ExchangeDone, NeighborPreviousState=Exchange, NeighborCurrentState=Loading)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:02-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[6]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=2.0.0.1, NeighborEvent=LoadingDone, NeighborPreviousState=Loading, NeighborCurrentState=Full)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:04-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[7]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:04-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[8]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:04-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[9]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=ExStart)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:05-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[10]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=NegotiationDone, NeighborPreviousState=ExStart, NeighborCurrentState=Exchange)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:05-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[11]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=ExchangeDone, NeighborPreviousState=Exchange, NeighborCurrentState=Loading)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:05-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[12]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=1.0.0.1, NeighborEvent=LoadingDone, NeighborPreviousState=Loading, NeighborCurrentState=Full)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:06-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[13]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=3.0.0.1, NeighborEvent=HelloReceived, NeighborPreviousState=Down, NeighborCurrentState=Init)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:06-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[14]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=3.0.0.1, NeighborEvent=2WayReceived, NeighborPreviousState=Init, NeighborCurrentState=2Way)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
Jul 24 2021 12:21:06-08:00 R4 %%01OSPF/4/NBR_CHANGE_E(l)[15]:Neighbor changes event: neighbor status changed. (ProcessId=256, NeighborAddress=3.0.0.1, NeighborEvent=AdjOk?, NeighborPreviousState=2Way, NeighborCurrentState=2Way)
[R4-ospf-1-area-0.0.0.0]dis ospf peer brief
OSPF Process 1 with Router ID 4.4.4.4
Peer Statistic Information
----------------------------------------------------------------------------
Area Id Interface Neighbor id State
0.0.0.0 GigabitEthernet0/0/0 1.1.1.1 Full
0.0.0.0 GigabitEthernet0/0/0 2.2.2.2 Full
0.0.0.0 GigabitEthernet0/0/0 3.3.3.3 2-Way
----------------------------------------------------------------------------
[R4-ospf-1-area-0.0.0.0]
提示：可以看到当R4配置好以后，对应R4和R1（DR），R2（BDR）是邻接关系；而和R3（DRothers）建立的邻居关系；
验证：在R1上发布更新，抓包看看对应的数据包会怎么发送和确认？
提示：可以看到当R1（DR）发送LSU时，它会直接向224.0.0.5发送；其中R2（BDR）向224.0.0.5确认；R3（DRothers）和R4（DRothers）向224.0.0.6确认；
验证：在R2上发送LSU，看看对应数据包会这么发送和确认的呢？
提示：可以看到当R2（BDR）发送LSU时，它和R1发送LSU一样直接向224.0.0.5发送LSU，对应R1（DR）直接向224.0.0.5确认；R3、R4（DRother）向224.0.0.6确认；
验证：当R3发送LSU，看看对应数据包会这么发送和确认呢？
提示：可以看到当R3（DRother）发送LSU时，它首先会把LSU通过224.0.0.6向R1（DR）发送，当R1（DR）收到R3发送的LSU，它会通过224.0.0.5向其他路由器发送LSU（不会再发送给R3）；R2（BDR）收到LSU向224.0.0.5确认，R4（DRother）向224.0.0.6确认；
验证：当R4 发送LSU时，看看对应数据包会怎么发送和确认呢？
提示：可以看到R4（DRother）发送LSU和R3（DRother）发送LSU一样；首先它会把LSU发送给R1（DR），然后由R1（DR）再向其他路由器发送LSU；BDR（R2）直接向224.0.0.5确认；DRother（R3）向224.0.0.6确认；
验证：全网重启ospf进程，看看对应的DR和BDR是否会重新选举？
提示：可以看到4个路由器分别重新启动了ospf以后，重置ospf 进程以后，R4当选为DR，R3当选为BDR；因为重启ospf进程对应的邻居和邻接关系又重新建立，此时4个路由器首先会比较对应的hello包里的优先级，默认情况下优先级都是1 ，此时优先级一样，会比较对应接口的ip地址谁大谁就是DR，次大者就为BDR，剩下没有被选举成DR或BDR的就沦为DRother；R4的router id 为4.4.4.4，是4个路由器中最大的，所以R4当选DR，R3的router id为3.3.3.3 ，第二大，所以当选BDR；
