前文我們了解了OSPF建立鄰居關系的條件，回顧請參考https://www.cnblogs.com/qiuhom-1874/p/15032907.html；今天我們來聊一聊OSPF中的DR和BDR；
在說DR和BDR之前我們先來了解下OSPF的網絡類型；
首先OSPF的網絡類型是基於接口來划分，其次OSPF的網絡類型是通過接口的二層封裝報文的方式進行了划分，默認情況下OSPF把接口二層是以太網封裝的網絡稱為廣播類型，把接口二層是PPP或者HDLC封裝的網絡稱為點到點類型；
在生活中，廣播就是一個人說話，在同一個房間里的其他人都可以聽到；在網絡里，廣播和這個類似；一個網絡設備在一個廣播域發送一條廣播報文，在同一個廣播域的所有設備都能收到該設備的廣播；這樣一來我們要想和其中的一台設備通訊怎么通訊呢？在二層為以太網封裝的廣播類型網絡里，它是通過二層以太網封裝目標MAC和源MAC；其原因是ip地址是變化的，如果單純的使用ip地址不能夠准確的標識某一台設備（因為ip地址是變化的），而MAC地址是設備的硬件地址，這個地址是設備出廠時就固定了，況且不會發生變化；所以確定一台設備的唯一性，我們可以通過MAC地址；有了通信雙方的mac地址再結合三層的ip地址，我們就可以准確的將對應數據包發送給對方；
對於點到點類型的網絡，其二層就不需要封裝MAC地址，一條鏈路上就只有兩個端點，在A端點發送數據B端點就一定能收到；
提示：廣播類型的接口是以太網類型接口（就是我們常常看到的網口），而點到點類型的接口是串口；
在點對點類型的網絡中，基於接口封裝的不同又可以分非廣播多路訪問和點到多點，如下圖
提示：默認情況下，OSPF認為幀中繼（FR）、異步傳輸（ATM）的網絡類型為NBMA；在幀中繼網絡里非廣播多路訪問和以太網中的廣播類型，兩者有一個相同的特點，就是都可以接多個設備；不同點是以太網二層封裝需要封裝mac地址，支持廣播發送，而幀中繼網絡二層不需要封裝mac地址，其原因是它不支持廣播；不支持廣播就意味着在ospf里不支持組播；不支持組播也就是說幀中繼網絡里，ospf只能通過單播的方式建立鄰居；
實驗：如下圖兩個拓撲，R1和R2為以太網類型接口，R3和R4為串口接口，在兩個實驗環境中配置ospf，抓包看看兩者發送到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：默認情況串口鏈路的二層封裝為ppp類型，這里抓包就選擇ppp鏈路類型即可；
提示：從上面的抓包情況可以看到，在以太網封類型的網絡環境下抓包，其二層封裝了目標mac和源mac，而在點到點類型的網絡環境下抓包，其二層就不是使用以太網協議而是用ppp協議，並且數據包里面也沒有封裝mac地址；
驗證：查看對應ospf網絡類型
在R1或R2上驗證相關接口的網絡類型
在R3或R4上驗證相關接口的網絡類型
更改R3的s4/0/0的鏈路封裝為FR，看看對應的接口類型是什么
提示：可以看到修改了R3的s4/0/0接口的封裝為FR以后，對應ospf就down掉了；並且對應接口的類型就變為了NBMA（非廣播多路訪問）；
OSPF DR和BDR
DR和BDR的作用
在上述的網絡類型中，在多路訪問網絡類型中跑OSPF，每個路由器都會和其他路由器建立鄰居，一個路由器發送一條鏈路更新，其他路由器都會跟着確認，這樣一來，每個路由器都要經過兩兩的交互和確認，在一定程度上會帶來路由器的性能的開銷（hello包，DD，LSR，LSU，LSACK這些包過多，增加路由器的處理壓力，從而導致路由器的性能降低）；為了降低數據包的交換，在OSPF中會選舉DR和BDR；如下圖
提示：廣播型網絡BMA和NBMA網絡中，為了減少鄰接關系的數量，從而減少數據包交換，次數，最終節省帶寬，降低對路由器處理能力的壓力，選舉DR和BDR。
相關術語概述
DR：Designed Router，指定路由器，類似班長、總經理；
BDR：Backup DR，備用DR，類似副班長、副總經理；
DRouthers：類似普通學生、普通員工；
三者的關系：DR、BDR、DRothers之間都保持鄰接關系（Full），DRothers之間保持鄰居關系（Two-Way）；
通信地址：224.0.0.6向DR和BDR發送鏈路狀態更新，224.0.0.5向所有OSPF路由器發送；
DR選舉規則
1、首先比較Hello報文中攜帶的優先級，優先級范圍是0-255，默認情況下都是1；優先級最高的被選舉為DR，優先級次高的被選舉為BDR；優先級為0的不參與選舉；
2、如果優先級相同的情況下，比較router id大小，router id越大越優先；
3、選舉不具有搶占性，除非當DR和BDR都失效或重啟OSPF進程；
提示：DRothers的LSU向DR或BDR通過224.0.0.6發送，然后DR收到DRothers發送到LSU以后，會將對應的LSU通過224.0.0.5向其他路由器發送，其他路由器收到DR發送LSU以后，BDR向224.0.0.5直接確認，而DBothers則通過224.0.0.6向DR確認；總結一句話就是從DR或BDR發出的數據包都是發往224.0.0.5，DR和BDR都會監聽224.0.0.6這個地址；DRothers發出的數據會通過224.0.0.6向DR或BDR發送；
實驗：如下圖，全網運行ospf，看看對應的數據包會這么發送，對應路由器之間是什么關系
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，看看誰是DR和BDR
提示：可以看到只配置了R1上的OSPF以后，對應DR就是它自己，因為此時沒有其他設備和它競選DR，所以只有它是DR；對於BDR來說，它不知道，因為目前就只有它自己在ospf里面，沒有看到其他設備；
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：可以看到當R2配置好ospf以后，它會和R1先建立其鄰接關系；
再次在R1上查看g0/0/0的接口信息，看看此時誰是DR和BDR
提示：此時可以看到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：可以看到當R3配置好以后，對應R3和R1和R2都建立起鄰接關系；
查看R3g0/0/0接口信息，看看DR和BDR變化沒有？
提示：之所以DR和BDR沒有發生變化，其原因是ospf已經選好了DR和BDR，而DR和BDR的選舉是非搶占性，一旦選好，就不變了，除非重啟ospf進程（所有路由器）來觸發DR和BDR的重新選舉；
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：可以看到當R4配置好以后，對應R4和R1（DR），R2（BDR）是鄰接關系；而和R3（DRothers）建立的鄰居關系；
驗證：在R1上發布更新，抓包看看對應的數據包會怎么發送和確認？
提示：可以看到當R1（DR）發送LSU時，它會直接向224.0.0.5發送；其中R2（BDR）向224.0.0.5確認；R3（DRothers）和R4（DRothers）向224.0.0.6確認；
驗證：在R2上發送LSU，看看對應數據包會這么發送和確認的呢？
提示：可以看到當R2（BDR）發送LSU時，它和R1發送LSU一樣直接向224.0.0.5發送LSU，對應R1（DR）直接向224.0.0.5確認；R3、R4（DRother）向224.0.0.6確認；
驗證：當R3發送LSU，看看對應數據包會這么發送和確認呢？
提示：可以看到當R3（DRother）發送LSU時，它首先會把LSU通過224.0.0.6向R1（DR）發送，當R1（DR）收到R3發送的LSU，它會通過224.0.0.5向其他路由器發送LSU（不會再發送給R3）；R2（BDR）收到LSU向224.0.0.5確認，R4（DRother）向224.0.0.6確認；
驗證：當R4 發送LSU時，看看對應數據包會怎么發送和確認呢？
提示：可以看到R4（DRother）發送LSU和R3（DRother）發送LSU一樣；首先它會把LSU發送給R1（DR），然后由R1（DR）再向其他路由器發送LSU；BDR（R2）直接向224.0.0.5確認；DRother（R3）向224.0.0.6確認；
驗證：全網重啟ospf進程，看看對應的DR和BDR是否會重新選舉？
提示：可以看到4個路由器分別重新啟動了ospf以后，重置ospf 進程以后，R4當選為DR，R3當選為BDR；因為重啟ospf進程對應的鄰居和鄰接關系又重新建立，此時4個路由器首先會比較對應的hello包里的優先級，默認情況下優先級都是1 ，此時優先級一樣，會比較對應接口的ip地址誰大誰就是DR，次大者就為BDR，剩下沒有被選舉成DR或BDR的就淪為DRother；R4的router id 為4.4.4.4，是4個路由器中最大的，所以R4當選DR，R3的router id為3.3.3.3 ，第二大，所以當選BDR；