一、概述：
EIGRP启用认证，用于防止恶意路由进入EIGRP自治系统导致路由攻击。EIGRP认证原理采用“密码比对”的形式，在接口受到EIGRP数据包后，检查认证类型、认证密码等字段，如果与本地接口定义的密码串匹配，那么接收该数据包，否则丢弃。
二：认证方式：
EIGRP有两种认证方式：明文认证与MD5认证，这里我们不推荐使用明文，明文是直接发送字符串给对方之后对比，容易被人通过抓包截获,而MD5是通过计算之后得出的值发送给对方对比,这次为大家做的为MD5实验.
实验：在实验的过程当中会为大家演示一些故障，使大家能够更好的理解.
拓扑如下：
步骤一：配置接口信息及启用EIGRP发布网段,并且建立邻居关系.
注：由于字数限制8万，EIGRP的基本配置没有写上来.
R2 查看EIGRP邻居表：
r2#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 192.168.1.1 Et0/0 10 00:02:28 3 200 0 2
r2#
步骤二：两边启用MD5认证，但是KEY-ID不一样，邻居是否能建立，为什么? （注：故障）
R1:
r1#
r1#conf t
r1(config)#key chain eigrp //定义钥匙链eigrp（名字任意取）
r1(config-keychain)#key 1 //KEY ID为1
r1(config-keychain-key)#key-string cisco //密码为cisco
r1(config-keychain-key)#exit
r1(config-keychain)#exit
r1(config)#interface e0/0
r1(config-if)#ip authentication key-chain eigrp 1 eigrp //启用EIGRP认证
r1(config-if)#ip authentication mode eigrp 1 md5 //认证模式MD5
r1(config-if)#end
r1#
R2:
r2#
r2#conf t
r2(config)#key chain eigrp
r2(config-keychain)#key 2 //KEY ID为2
r2(config-keychain-key)#key-string cisco
r2(config-keychain-key)#exit
r2(config-keychain)#exit
r2(config)#interface e0/0
r2(config-if)#ip authentication key-chain eigrp 1 eigrp
r2(config-if)#ip authentication mode eigrp 1 md5
r2(config-if)#end
r2#
查看邻居表：
r2#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
r2#
//说明没有建立邻居关系，为什么？
查看Debug:
r2#debug eigrp packets
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)
*Mar 1 22:31:40.962: EIGRP: Sending HELLO on Ethernet0/0
*Mar 1 22:31:40.962: AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Mar 1 22:31:41.187: EIGRP: pkt authentication key id = 1, key not defined or not live
//这里清楚的看到，收到的KEY ID = 1 ，key没有定义或不存在,因为们的KEY ID为2
*Mar 1 22:31:41.187: EIGRP: Ethernet0/0: ignored packet from 192.168.1.1, opcode = 5 (invalid authentication)
步骤三：双方KEY ID一致，但密码不一致，邻居关系是否能建立，为什么？ （注：故障）
R2:
r2#
r2#conf t
r2(config)#key chain eigrp
r2(config-keychain)#key 1
r2(config-keychain-key)#key-string cisco1 //这里密码是cisco1，与r1不一致
r2(config-keychain-key)#end
r2#
查看邻居表：
r2#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
r2#
//还是一样没有建立，为什么？
查看Debug:
r2#debug eigrp packets
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)
r2#
*Mar 1 22:46:14.094: EIGRP: pkt key id = 1, authentication mismatch
//这里没有显示KEY ID不正确，而是认证不匹配，说明密码不一致.
*Mar 1 22:46:14.094: EIGRP: Ethernet0/0: ignored packet from 192.168.1.1, opcode = 5 (invalid authentication)
步骤四：双方KEY ID一样，密码也一样,可以正常建立邻居关系.
R1：
r1(config)#
r1(config)#key chain eigrp
r1(config-keychain)#key 1
r1(config-keychain-key)#key-string cisco
r1(config)#interface e0/0
r1(config-if)#ip authentication key-chain eigrp 1 eigrp
r1(config-if)#ip authentication mode eigrp 1 md5
r1(config-keychain-key)#end
r2#
R2:
r2(config)#
r2(config)#key chain eigrp
r2(config-keychain)#key 1
r2(config-keychain-key)# key-string cisco
r2(config)#interface e0/0
r2(config-if)#ip authentication key-chain eigrp 1 eigrp
r2(config-if)#ip authentication mode eigrp 1 md5
*Mar 1 22:51:33.037: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.1.1 (Ethernet0/0) is up: new adjacency //正常建立，OK！！
r2(config-keychain-key)#end
r2#
步骤五：分别在R1与R2上验证
R1:
查看邻居表：
r1#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 192.168.1.2 Et0/0 13 00:03:00 1321 5000 0 9
r1#
查看路由表：
r1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/409600] via 192.168.1.2, 00:03:23, Ethernet0/0
C 192.168.1.0/24 is directly connected, Ethernet0/0
r1#
------------------------------------------------------------------------------------
------------------------------------------------------------------------------------
R2:
查看邻居表：
r2#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 192.168.1.1 Et0/0 12 00:01:17 20 200 0 8
r2#
查看路由表：
r2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/409600] via 192.168.1.1, 00:01:41, Ethernet0/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
C 192.168.1.0/24 is directly connected, Ethernet0/0
r2#
总结（认证过程）：
只发送最小的一个KEY-ID的密钥匙.并携带KEY=ID.当本路由器接受到对方发过来的KEY-ID和密钥时.先匹配收到的密钥的KEY-ID号与自己的KEY-ID号是否相同.如果KEY-ID号有相同的,则看密码是否相同,密码相同则认证通过.密码不同认证不通过.如果对方发过来的KEY-ID号自己没有相同的KEY-ID号,就向下一个匹配一个KEY-ID号中的密钥(既比收到的KEY-ID号大的的下一个KEY-ID),而且只向下匹配一次KEY-ID号中的密钥.既密文认证是先匹配KEY-ID在匹配KEY-ID的内容.