邻居认证使得路由器确认每次所收到的路由更新的源。如果关键字不匹配，就会拒绝路由更新。
Cisco使用两种类型的邻居认证：纯文本和MD5。
纯文本认证发一个关键字，这个关键字是明文传输，可被非法用户所窃取，所以不推荐使用。
MD5认证
发一个报文摘要，而不是关键字。MD5被用来生成一个关键字的散列。这个散列是被发送的对象。MD5方式不易被非法用户所窃取。
这个案例中，我们在R1与R2之间使用明文认证，在R2与R3之间使用MD5认证。
// R1 //
int e0/0
ip ad 192.1.1.1 255.255.255.0
ip ospf authentication-key cisco //明文认证，关键字为cisco
router os 1
network 192.1.1.1 0.0.0.0 area 0
area 0 authentication
// R2 //
int e0/0
ip ad 192.1.1.2 255.255.255.0
ip ospf authentication-key cisco //明文认证，关键字为cisco
int e1/0
ip ad 193.1.1.2 255.255.255.0
ip ospf message-digest-key 1 md5 cracker
router os 1
network 192.1.1.2 0.0.0.0 area 0
network 193.1.1.2 0.0.0.0 area 1
area 0 authentication
area 1 authentication message-digest
// R3 //
int e1/0
ip ad 193.1.1.3 255.255.255.0
ip ospf message-digest-key 1 md5 cracker
router os 1
network 193.1.1.3 0.0.0.0 a 1
area 1 authentication message-digest
验证过程：
r1#sh ip os int e0/0
Ethernet0/0 is up, line protocol is up
Internet Address 192.1.1.1/24, Area 0
Process ID 1, Router ID 192.1.1.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 193.1.1.2, Interface address 192.1.1.2
Backup Designated router (ID) 192.1.1.1, Interface address 192.1.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:06
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 193.1.1.2 (Designated Router)
Suppress hello for 0 neighbor(s)
Simple password authentication enabled
r2#sh ip os int e0/0
Ethernet0/0 is up, line protocol is up
Internet Address 192.1.1.2/24, Area 0
Process ID 1, Router ID 193.1.1.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 193.1.1.2, Interface address 192.1.1.2
Backup Designated router (ID) 192.1.1.1, Interface address 192.1.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
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 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.1.1.1 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Simple password authentication enabled
r2#sh ip os int e1/0
Ethernet1/0 is up, line protocol is up
Internet Address 193.1.1.2/24, Area 1
Process ID 1, Router ID 193.1.1.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 193.1.1.2, Interface address 193.1.1.2
Backup Designated router (ID) 193.1.1.3, Interface address 193.1.1.3
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03
Index 1/2, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 2, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 193.1.1.3 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
Youngest key id is 1
r3#sh ip os int e1/0
Ethernet1/0 is up, line protocol is up
Internet Address 193.1.1.3/24, Area 1
Process ID 1, Router ID 193.1.1.3, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 193.1.1.2, Interface address 193.1.1.2
Backup Designated router (ID) 193.1.1.3, Interface address 193.1.1.3
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
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 1, Adjacent neighbor count is 1
Adjacent with neighbor 193.1.1.2 (Designated Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
Youngest key id is 1
为了更进一步理解认证过程，我们可以打开DEBUG，并将R3的MD5认证key改为5：
// R3 //
debug ip ospf adj
int e1/0
ip ospf message-digest-key 5 md5 cracker
r3#
01:16:03: OSPF: Rcv pkt from 193.1.1.2, Ethernet1/0 : Mismatch Authentication Key - No message digest key 1 on interface
01:16:09: OSPF: Send with youngest Key 5
r3#show ip ospf neighbor //观察结果无法发现邻居。
//认证未通过，无法与R2建立起邻居关系。
当我们把MD5认证KEY改回1后，认证通过。
第二步实验，我们把关键字进行修改：
// R3 //
debug ip ospf adj
int e1/0
ip ospf message-digest-key 1 md5 cuijian
01:21:33: OSPF: Rcv pkt from 193.1.1.2, Ethernet1/0 : Mismatch Authentication Key - Message Digest Key 1
01:21:40: OSPF: Send with youngest Key 1
我们要在实际工作中学会使用debug这个思科排错的利器。
出处：Cisco网站