ip default-gateway、ip route和ip default-network的区别
指定默认路由（last resort gateway）的指令供有3种，可以分成两类：
1、ip default-gateway
当路由器上的ip routing无效时，使用它指定默认路由，用于RXBoot模式（no ip routing）下安装IOS等。或者关闭ip routing 让路由器当主机用,此时需要配置默认网关。
2、ip default-network和ip route 0.0.0.0 0.0.0.0
两者都用于ip routing有效的路由器上，区别主要在于路由协议是否传播这条路由信息。比如：IGRP无法识别0.0.0.0，因此传播默认路由时必须用ip default-network。
当用ip default-network指令设定多条默认路由时，administrative distance最短的成为最终的默认路由；如果有复数条路由distance值相等，那么在路由表（show ip route）中靠上的成为默认路由。
同时使用ip default-network和ip route 0.0.0.0 0.0.0.0双方设定默认路由时，如果ip default-network设定的网络是直连（静态、且已知）的，那么它就成为默认路由；如果ip default-network指定的网络是由交换路由信息得来的，则ip route 0.0.0.0 0.0.0.0指定的表项成为默认路由。
最后，如果使用多条ip route 0.0.0.0 0.0.0.0指令，则流量会自动在多条链路上负载均衡。
//ip default-gateway测试
Mycisco(config)#no ip routing
Mycisco(config)#ip default-gateway 10.1.2.1
Mycisco(config)#do ping 119.75.217.56
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 119.75.217.56, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/217/1040 ms
Mycisco(config)#do sh ip route
Default gateway is 10.1.2.1
Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty
Mycisco(config)#
//ip route测试
Mycisco(config)#ip route 0.0.0.0 0.0.0.0 10.1.2.1
Mycisco(config)#do ping 119.75.217.56
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 119.75.217.56, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/20/44 ms
Mycisco(config)#do sh 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, 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 10.1.2.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 10.1.2.1
Mycisco(config)#
//ip default-network测试 (ip default-network 必须是在所到网络已经存在路由的情况下,否则执行无效).
R2(config)#ip route 119.0.0.0 255.0.0.0 172.16.0.1
R2(config)#do sh 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, 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 not set
S 119.0.0.0/8 [1/0] via 172.16.0.1
C 172.16.0.0/16 is directly connected, FastEthernet0/0
R2(config)#ip default-network 119.0.0.0
R2(config)#do sh 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, 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 172.16.0.1 to network 119.0.0.0
S* 119.0.0.0/8 [1/0] via 172.16.0.1
C 172.16.0.0/16 is directly connected, FastEthernet0/0
R2(config)#