# DR：指定路由器# BDR：备份指定路由器# DROTHER：非DR/BDR的其他路由器# DR/BDR选举过程# MA网络用途：选举DR，用于可以减少数据库条目，减少网络开销# 1、wait timer （ dead timer ) 40s等待时间，若无hello包，自己成为DR；# 2、priority（接口中ospf优先级，0-255，优先级增大）；# 3、priority相同，比较Router-id，优先级越大；# 4、DR/BDR的选举过程为非抢占的# 5、priority为0，不能成为DR/BDR，DROTHER之间为2WAY状态；IOU3#sh ip ospf neighborNeighbor ID Pri State Dead Time Address Interface1.1.1.1 1 FULL/DROTHER 00:00:37 10.1.123.1 Ethernet0/02.2.2.2 1 FULL/BDR 00:00:37 10.1.123.2 Ethernet0/0IOU3#sh ip ospf neighborNeighbor ID Pri State Dead Time Address Interface1.1.1.1 0 FULL/DROTHER 00:00:34 10.1.123.1 Ethernet0/02.2.2.2 0 FULL/DROTHER 00:00:35 10.1.123.2 Ethernet0/0IOU1#sh ip ospf neighborNeighbor ID Pri State Dead Time Address Interface2.2.2.2 0 2WAY/DROTHER 00:00:33 10.1.123.2 Ethernet0/03.3.3.3 255 FULL/DR 00:00:32 10.1.123.3 Ethernet0/0IOU2#sh ip ospf neighborNeighbor ID Pri State Dead Time Address Interface1.1.1.1 0 2WAY/DROTHER 00:00:38 10.1.123.1 Ethernet0/03.3.3.3 255 FULL/DR 00:00:31 10.1.123.3 Ethernet0/0IOU3#sh ip ospf int e0/0Ethernet0/0 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 e0/0Ethernet0/0 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 监听组播地址：224.0.0.5 224.0.0.6
# OSPF 邻居建立过程# 报文5种类型：HELLO、DBD、LSR、LSU、LSACK# 邻居状态：down、init、2way、exstart、exchange、loading、full、attempty(NBMA/P2MP)# 1、R4发送Hello至224.0.0.5；# 2、R5收到Hello包后，邻居状态设置为init，并回复Hello，携带active neibour：R4的router-id；# 3、R4收到携带active neibour的Hello包后，设置R5的邻居状态为2way，并回复Hello，携带active neibour：R5的router-id；# 4、R5收到携带active neibour的Hello包后，设置R4的邻居状态为2way;假设R5为DR或者为Priority/router-id较大；# 开始同步DBD（数据库描述，LSA头部）和LSR一起用于提高OSPF数据库同步效率，同步过程中不使用ack，可靠有序；# 可靠有序机制：根据router-id选举主从机制，从确认主，非累加确认；# 5、主从的第一个交互DBD不携带信息，为空，邻居状态为exstart；# 从先发序列号4321，主发送序列号1234，从用主的序列号1234确认主（序列号非累加），主确认从在发送1235（序列号累加）；# 6、除第一个DBD交互过程，邻居状态为exchange；# flag标志位：I/M/M(S)，是否为第一个DBD/是否DBD剩余/主从；从的flag为0x000，主flag为0x001；# 特殊的DBD交互过程：从的DBD先完/主的DBD先完；# DBD交互完毕后，从发送LSR,主发送LSU，并发送LSRACK进行确认；可靠有序