实验拓扑：
接口连接：
Router1 S0/0 <----> Router2 S0/0
Router2 S0/1 <----> Router3 S0/0
Router1 S0/1 <----> Router4 S0/0
Router3 S0/1 <----> Router4 S0/1
VPCS V0/1 <----> Router1 E1/0
VPCS V0/2 <----> Router3 E1/0
实验原理：
在R1\R2\R3\R4上均运行EIGRP路由协议。R4通过S0/1向R3通告一条0.0.0.0 0.0.0.0的汇总路由，并将这条浮动路由的管理距离设置为250，于是在网络连通的情况下R3上到达200.1.1.0/24网段的路由经过R2，而 在R2与R3链路故障的时候，通过R3前往200.1.1.0/24的数据包会通过R4（R3拓扑表中，默认路由由A变为P）
关键配置：
R1:
interface Serial0/0
ip address 192.168.1.1 255.255.255.0
serial restart-delay 0
!
interface Serial0/1
ip address 192.168.3.1 255.255.255.0
serial restart-delay 0
!
interface Ethernet1/0
ip address 200.1.1.254 255.255.255.0
half-duplex
!
router eigrp 100
network 192.168.1.0
network 192.168.3.0
network 200.1.1.0
no auto-summary
R2、R3略，参照R1进行接口IP和路由协议的配 置
重点看下R4的配置
R4:
interface Serial0/0
ip address 192.168.3.2 255.255.255.0
serial restart-delay 0
!
interface Serial0/1
ip address 192.168.4.1 255.255.255.0
ip summary-address eigrp 100 0.0.0.0 0.0.0.0 250（向R3通过默认路由，管理距离高于EIGRP默认值）!
router eigrp 100
network 192.168.3.0
network 192.168.4.0
auto-summary
查看结果：
1.R2和R3之间链路正常时
a) R3的路由表：
R3#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, su - IS-IS summary, 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 192.168.4.1 to network 0.0.0.0
D 200.1.1.0/24 [90/2707456] via 192.168.2.1, 00:00:02, Serial0/0
C 200.1.2.0/24 is directly connected, Ethernet1/0
C 192.168.4.0/24 is directly connected, Serial0/1
D 192.168.1.0/24 [90/2681856] via 192.168.2.1, 00:00:02, Serial0/0
C 192.168.2.0/24 is directly connected, Serial0/0
D 192.168.3.0/24 [90/3193856] via 192.168.2.1, 00:00:02, Serial0/0
D* 0.0.0.0/0 [90/2681856] via 192.168.4.1, 00:00:00, Serial0/1
b)R3的拓扑表：
R3#sh ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(200.1.2.254)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
A 0.0.0.0/0, 1 successors, FD is Inaccessible
1 replies, active 00:00:00, query-origin: Successor Origin
Remaining replies:
via 192.168.2.1, r, Serial0/0
P 192.168.1.0/24, 1 successors, FD is 2681856
via 192.168.2.1 (2681856/2169856), Serial0/0
P 192.168.2.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
P 192.168.3.0/24, 1 successors, FD is 3193856
via 192.168.2.1 (3193856/2681856), Serial0/0
P 192.168.4.0/24, 1 successors, FD is 2169856
via Connected, Serial0/1
P 200.1.1.0/24, 1 successors, FD is 2707456
via 192.168.2.1 (2707456/2195456), Serial0/0
P 200.1.2.0/24, 1 successors, FD is 281600
via Connected, Ethernet1/0
2.R2的S0/1设置为DOWN,观察R3路由表和拓扑表
a)R3路由表：
R3#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, su - IS-IS summary, 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 192.168.4.1 to network 0.0.0.0
C 200.1.2.0/24 is directly connected, Ethernet1/0
C 192.168.4.0/24 is directly connected, Serial0/1
C 192.168.2.0/24 is directly connected, Serial0/0
D* 0.0.0.0/0 [90/2681856] via 192.168.4.1, 00:00:05, Serial0/1
b)R3的拓扑表：
R3#sh ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(200.1.2.254)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 0.0.0.0/0, 1 successors, FD is 2681856
via 192.168.4.1 (2681856/2169856), Serial0/1
P 192.168.4.0/24, 1 successors, FD is 2169856
via Connected, Serial0/1
P 200.1.2.0/24, 1 successors, FD is 281600
via Connected, Ethernet1/0
该实验说明，当R2和R3链路正常是，通过R3去往PC1的数据包不通过R4，只有当R2、R3间链路出现故障，默认路由才启用。
