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【實驗目的】
了解和掌握OSPF的原理，熟悉OSPF配置步驟。懂得如何配置OSPF router ID，update timers, authentication,了解DR/BDR選舉過程。

【基本概念及實驗原理】
? OSPF數據包的五種類型：
Type 1- Hello
用于建立和維持與鄰居的連接信息
Type 2- Database description packet(DBD)
用于描述一個路由器的鏈路狀態數據庫的內容
Type 3- Link-state request(LSR)
用于請求一個路由器鏈路狀態數據庫的一些特定的條目
Type 4- Link-state update(LSU)
用于把“鏈路狀態更新”(LSAs)傳輸給其它路由器
Type 5- Link-state acknowledgment (LSAck)
用于確認自己收到了一個從鄰居發過來的

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【實驗內容】
一、? multi-access網絡的OSPF配置
1、配置各臺路由器的端口地址，并利用ping命令檢查連通性
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端口作為OSPF router ID，確保router ID的穩定性。
Router(config)#Interface loopback 0
Router(config-if)#ip address 192.168.*.* 255.255.255.255
3、啟動router上OSPF進程并觀察
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被選舉為DR，R2為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時間間隔有相等的要求，
所以在先改動了一臺router參數后可觀察到握手不成功。

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與R3斷開連接

00:21:05: OSPF: DR/BDR election on FastEthernet0/0

00:21:05: OSPF: Elect BDR 192.168.11.11???????????????? //R1被選為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與R3斷開連接

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與R2的連接還在，重新選舉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?為DR，R1為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

此時會再次引發選舉，DR恢復原狀

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

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重新選舉后DR依舊是R3，而BDR卻變為了R1而不是ROUTER ID高的R2.這是因為在R3斷開之后R2成為DR，R1成為BDR，重新選舉時,R2宣告自己為DR，因此不能宣告BDR，而R2的router id要比R3低，而R1宣告自己為BDR。

參見：

1． 在與一個或多個鄰居之間的雙向通信建立起來之后，（路由器）對每個鄰居（發送來）的Hello包中的優先級、DR和BDR域進行檢查。列出所有能夠參加選舉的路由器（也就是說，路由器的優先級高于0并且此路由器的鄰居狀態至少為“雙向”）；所有路由器都宣稱自己為DR（將它們自己的接口地址置于Hello包的DR域中）；而且所有路由器都宣稱自己為BDR（將它們自己的接口地址置于Hello包的BDR域中）。進行計算的路由器也要將自身包括在此列表內，除非它被禁止參加選舉。
2． 從（以上）備選路由器列表中，創造一個子集，此子集包含所有未宣稱為DR的路由器（宣稱自己為DR的路由器無法被選舉為BDR）.
3． 如果此子集中的一或多個鄰居將它（們）自身的接口地址置于BDR域中，這些鄰居中擁有最高優先級的路由器將被宣告為BDR。如果出現平局（路由器優先級相等），擁有最高Router ID的鄰居將被選舉出來。
4． 如果此子集中沒有任何路由器被宣告為BDR，擁有最高優先級的鄰居將被宣告為BDR。如果出現平局，擁有最高Router ID的鄰居將被選舉出來。
5． 如果一或多個備選路由器將它（們）自身的接口地址置于DR域中，擁有最高優先級的鄰居將被宣告為DR。如果出現平局，擁有最高Router ID的鄰居將被選舉出來。
6． 如果沒有任何路由器宣告自己為DR，則新選舉出來的BDR將成為DR。
7． 如果進行計算的路由器是新選舉出來的DR或者BDR，或者如果它不再是DR或者BDR，重復步驟2到6。

簡而言之，當一個OSPF路由器啟動并開始搜索鄰居時，它先搜尋活動的DR和BDR。如果DR和BDR存在，路由器就接受它們。如果沒有BDR，就進行一次選舉將擁有最高優先級的路由器選舉為BDR。如果多于一臺路由器擁有相同的優先級，那么擁有最高路由器ID的路由器將勝出。如果沒有活動的DR，BDR將被提升為DR然后再進行一次BDR的選舉。

以上譯自《Routing TCP/IP Volume I》 Jeff Doyle Cisco Press 1998 / PTP 2003

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轉載于:https://blog.51cto.com/ciscowu/596491

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