【实验目的】
了解和掌握OSPF的原理，熟悉OSPF配置步骤。懂得如何配置OSPF router ID，update timers, authentication,了解DR/BDR选举过程。
【基本概念及实验原理】
OSPF数据包的五种类型：
Type 1- Hello
用于建立和维持与邻居的连接信息
Type 2- Database description packet(DBD)
用于描述一个路由器的链路状态数据库的内容
Type 3- Link-state request(LSR)
用于请求一个路由器链路状态数据库的一些特定的条目
Type 4- Link-state update(LSU)
用于把“链路状态更新”(LSAs)传输给其它路由器
Type 5- Link-state acknowledgment (LSAck)
用于确认自己收到了一个从邻居发过来的
【实验内容】
一、 multi-access网络的OSPF配置
1、配置各台路由器的端口地址，并利用ping命令检查连通性
Router(config)#Interface Ethernet 0
Router(config-if)#ip address 192.168.1.* 255.255.255.*
Router(config-if)#no shutdown
2、配置loopback端口作为OSPF router ID，确保router ID的稳定性。
Router(config)#Interface loopback 0
Router(config-if)#ip address 192.168.*.* 255.255.255.255
3、启动router上OSPF进程并观察
Router(config)#router ospf 1
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0
=====================================================
二、查看OSPF NEIGHBOR
r3#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.22.22 1 FULL/BDR 00:00:31 192.168.1.2 FastEthernet0/0
192.168.11.11 1 FULL/DROTHER 00:00:32 192.168.1.1 FastEthernet0/0
r3#sh ip ospf neighbor detail
Neighbor 192.168.22.22, interface address 192.168.1.2
In the area 0 via interface FastEthernet0/0
Neighbor priority is 1, State is FULL, 5 state changes
DR is 192.168.1.3 BDR is 192.168.1.2 //R3被选举为DR，R2为BDR
Options is 0x00
Dead timer due in 00:00:31
Neighbor is up for 00:06:39
Index 1/1, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
Neighbor 192.168.11.11, interface address 192.168.1.1
In the area 0 via interface FastEthernet0/0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 192.168.1.3 BDR is 192.168.1.2
Options is 0x00
Dead timer due in 00:00:32
Neighbor is up for 00:06:28
Index 2/2, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 2
Last retransmission scan time is 0 msec, maximum is 0 msec
三、更改timer
R3(config-if)#ip ospf hello-interval 30
R3(config-if)#ip ospf dead-interval 60
由于hello时间间隔有相等的要求，
所以在先改动了一台router参数后可观察到握手不成功。
r3#debug ip ospf event
OSPF events debugging is on
r3#
00:21:05: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.22.22 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired
00:21:05: OSPF: Neighbor change Event on interface FastEthernet0/0
00:21:05: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.22.22 on FastEthernet0/0 from FULL to Down: Interface down or detached //R2与R3断开连接
00:21:05: OSPF: DR/BDR election on FastEthernet0/0
00:21:05: OSPF: Elect BDR 192.168.11.11 //R1被选为BDR
00:21:05: OSPF: Elect DR 192.168.33.33
00:21:05: DR: 192.168.33.33 (Id) BDR: 192.168.11.11 (Id)
00:21:05: OSPF: Rcv hello from 192.168.22.22 area 0 from FastEthernet0/0 192.168.1.2
00:21:05: OSPF: Mismatched hello parameters from 192.168.1.2
00:21:05: OSPF: Dead R 40 C 60 Hello R 10 C 30 Mask R 255.255.255.0 C 255.255.255.0
00:21:05: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.11.11 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired
00:21:05: OSPF: Neighbor change Event on interface FastEthernet0/0
00:21:05: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.11.11 on FastEthernet0/0 from FULL to Down: Interface down or detached // //R2与R3断开连接
00:21:05: OSPF: DR/BDR election on FastEthernet0/0
00:21:05: OSPF: Elect BDR 0.0.0.0
00:21:05: OSPF: Elect DR 192.168.33.33
00:21:05: DR: 192.168.33.33 (Id) BDR: none
00:21:06: OSPF: Rcv hello from 192.168.11.11 area 0 from FastEthernet0/0 192.168.1.1
00:21:06: OSPF: Mismatched hello parameters from 192.168.1.1
00:21:06: OSPF: Dead R 40 C 60 Hello R 10 C 30 Mask R 255.255.255.0 C 255.255.255.0
00:21:15: OSPF: Rcv hello from 192.168.22.22 area 0 from FastEthernet0/0 192.168.1.2
00:21:15: OSPF: Mismatched hello parameters from 192.168.1.2
========================================
而R1与R2的连接还在，重新选举DR和BDR
r1#sh ip ospf neighbor detail
Neighbor 192.168.22.22, interface address 192.168.1.2
In the area 0 via interface FastEthernet0/0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 192.168.1.2 BDR is 192.168.1.1 //r2 为DR，R1为BDR
Options is 0x00
Dead timer due in 00:00:34
Neighbor is up for 00:17:47
Index 1/1, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
四、改回原DR的interval
此时会再次引发选举，DR恢复原状
R3(config-if)#ip ospf hello-interval 10
R3(config-if)#ip ospf dead-interval 40
r3#debug ip ospf events
OSPF events debugging is on
r3#
……………………
00:32:16: OSPF: End of hello processing
00:32:16: OSPF: Neighbor change Event on interface FastEthernet0/0
00:32:16: OSPF: DR/BDR election on FastEthernet0/0
00:32:16: OSPF: Elect BDR 192.168.11.11
00:32:16: OSPF: Elect DR 192.168.33.33
00:32:16: DR: 192.168.33.33 (Id) BDR: 192.168.11.11 (Id)
00:32:26: OSPF: Rcv hello from 192.168.22.22 area 0 from FastEthernet0/0 192.168.1.2
00:32:26: OSPF: End of hello processing
重新选举后DR依旧是R3，而BDR却变为了R1而不是ROUTER ID高的R2.这是因为在R3断开之后R2成为DR，R1成为BDR，重新选举时,R2宣告自己为DR，因此不能宣告BDR，而R2的router id要比R3低，而R1宣告自己为BDR。
参见：
1． 在与一个或多个邻居之间的双向通信建立起来之后，（路由器）对每个邻居（发送来）的Hello包中的优先级、DR和BDR域进行检查。列出所有能够参加选举的路由器（也就是说，路由器的优先级高于0并且此路由器的邻居状态至少为“双向”）；所有路由器都宣称自己为DR（将它们自己的接口地址置于Hello包的DR域中）；而且所有路由器都宣称自己为BDR（将它们自己的接口地址置于Hello包的BDR域中）。进行计算的路由器也要将自身包括在此列表内，除非它被禁止参加选举。
2． 从（以上）备选路由器列表中，创造一个子集，此子集包含所有未宣称为DR的路由器（宣称自己为DR的路由器无法被选举为BDR）.
3． 如果此子集中的一或多个邻居将它（们）自身的接口地址置于BDR域中，这些邻居中拥有最高优先级的路由器将被宣告为BDR。如果出现平局（路由器优先级相等），拥有最高Router ID的邻居将被选举出来。
4． 如果此子集中没有任何路由器被宣告为BDR，拥有最高优先级的邻居将被宣告为BDR。如果出现平局，拥有最高Router ID的邻居将被选举出来。
5． 如果一或多个备选路由器将它（们）自身的接口地址置于DR域中，拥有最高优先级的邻居将被宣告为DR。如果出现平局，拥有最高Router ID的邻居将被选举出来。
6． 如果没有任何路由器宣告自己为DR，则新选举出来的BDR将成为DR。
7． 如果进行计算的路由器是新选举出来的DR或者BDR，或者如果它不再是DR或者BDR，重复步骤2到6。
简而言之，当一个OSPF路由器启动并开始搜索邻居时，它先搜寻活动的DR和BDR。如果DR和BDR存在，路由器就接受它们。如果没有BDR，就进行一次选举将拥有最高优先级的路由器选举为BDR。如果多于一台路由器拥有相同的优先级，那么拥有最高路由器ID的路由器将胜出。如果没有活动的DR，BDR将被提升为DR然后再进行一次BDR的选举。
以上译自《Routing TCP/IP Volume I》 Jeff Doyle Cisco Press 1998 / PTP 2003
