# DR：指定路由器
# BDR：備份指定路由器
# DROTHER：非DR/BDR的其他路由器

# DR/BDR選舉過程
# 1、wait timer （ dead timer ) 40s等待時間，若無hello包，自己成為DR；
# 2、priority（接口中ospf優先級，0-255，優先級增大）；
# 3、priority相同，比較Router-id，優先級越大；
# 4、DR/BDR的選舉過程為非搶占的
# 5、priority為0，不能成為DR/BDR，DROTHER之間為2WAY狀態；

IOU3#sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DROTHER    00:00:37    10.1.123.1      Ethernet0/0
2.2.2.2           1   FULL/BDR        00:00:37    10.1.123.2      Ethernet0/0

IOU3#sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/DROTHER    00:00:34    10.1.123.1      Ethernet0/0
2.2.2.2           0   FULL/DROTHER    00:00:35    10.1.123.2      Ethernet0/0

IOU1#sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   2WAY/DROTHER    00:00:33    10.1.123.2      Ethernet0/0
3.3.3.3         255   FULL/DR         00:00:32    10.1.123.3      Ethernet0/0

IOU2#sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   2WAY/DROTHER    00:00:38    10.1.123.1      Ethernet0/0
3.3.3.3         255   FULL/DR         00:00:31    10.1.123.3      Ethernet0/0

IOU3#sh ip ospf int e0/0
Ethernet0/0 is up, line protocol is up 
  Internet Address 10.1.123.3/24, Area 2, Attached via Network Statement
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 10
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           10        no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 255
  Designated Router (ID) 3.3.3.3, Interface address 10.1.123.3
  Backup Designated router (ID) 2.2.2.2, Interface address 10.1.123.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:07
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 2
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 2, Adjacent neighbor count is 2 
    Adjacent with neighbor 1.1.1.1
    Adjacent with neighbor 2.2.2.2  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

IOU3#sh ip int e0/0
Ethernet0/0 is up, line protocol is up
  Internet address is 10.1.123.3/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.251 224.0.0.5 224.0.0.6  # DR/BDR 監聽組播地址：224.0.0.5 224.0.0.6

# OSPF 鄰居建立過程
# 報文5種類型：HELLO、DBD、LSR、LSU、LSACK
# 鄰居狀態：down、init、2way、exstart、exchange、loading、full、attempty(NBMA/P2MP)
# 1、R4發送Hello至224.0.0.5；
# 2、R5收到Hello包後，鄰居狀態設置為init，並回復Hello，攜帶active neibour：R4的router-id；
# 3、R4收到攜帶active neibour的Hello包後，設置R5的鄰居狀態為2way，並回復Hello，攜帶active neibour：R5的router-id；
# 4、R5收到攜帶active neibour的Hello包後，設置R4的鄰居狀態為2way;假設R5為DR或者為Priority/router-id較大；
# 開始同步DBD（數據庫描述，LSA頭部）和LSR一起用於提高OSPF數據庫同步效率，同步過程中不使用ack，可靠有序；
# 可靠有序機制：根據router-id選舉主從機制，從確認主，非累加確認；
# 5、主從的第一個交互DBD不攜帶信息，為空，鄰居狀態為exstart；
#    從先發序列號4321，主發送序列號1234，從用主的序列號1234確認主（序列號非累加），主確認從在發送1235（序列號累加）；
# 6、除第一個DBD交互過程，鄰居狀態為exchange；
# flag標誌位：I/M/M(S)，是否為第一個DBD/是否DBD剩余/主從；從的flag為0x000，主flag為0x001；
# 特殊的DBD交互過程：從的DBD先完/主的DBD先完；
# DBD交互完畢後，從發送LSR,主發送LSU，並發送LSRACK進行確認；可靠有序

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