在自己的2501上研究這三種默認路由的區(qū)別.

指定默認路由（last resort gateway）的指令供有3種，可以分成兩類：
1、ip default-gateway
當路由器上的ip routing無效時，使用它指定默認路由，用于RXBoot模式（no ip routing）下安裝IOS等。或者關閉ip routing 讓路由器當主機用,此時需要配置默認網(wǎng)關

2、ip default-network和ip route 0.0.0.0 0.0.0.0
兩者都用于ip routing有效的路由器上，區(qū)別主要在于路由協(xié)議是否傳播這條路由信息。比如：IGRP無法識別0.0.0.0，因此傳播默認路由時必須用ip default-network。

當用ip default-network指令設定多條默認路由時，administrative distance最短的成為最終的默認路由；假如有復數(shù)條路由distance值相等，那么在路由表（show ip route）中靠上的成為默認路由。
同時使用ip default-network和ip route 0.0.0.0 0.0.0.0雙方設定默認路由時，假如ip default-network設定的網(wǎng)絡是直連（靜態(tài)、且已知）的，那么它就成為默認路由；假如ip default-network指定的網(wǎng)絡是由交換路由信息得來的，則ip route 0.0.0.0 0.0.0.0指定的表項成為默認路由。
最后，假如使用多條ip route 0.0.0.0 0.0.0.0指令，則流量會自動在多條鏈路上負載均衡。

官方具體文檔點這里

例子:

關閉ip routing 舉例:
mycisco(config)#no ip routing
mycisco(config)#ip default-gateway 192.168.0.1
mycisco(config)#end
mycisco#ping www.Flashku.com

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echoes to 61.152.167.75, timeout is 2 seconds:
!!!!!
SUCcess rate is 100 percent (5/5), round-trip min/avg/max = 60/60/60 ms
mycisco#show ip route
Default gateway is 192.168.0.1

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty
mycisco#

ip route例子:
ip route 0.0.0.0 0.0.0.0 192.168.0.1

mycisco#ping www.flashku.com

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echoes to 61.152.167.75, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/60 ms
mycisco#show 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
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
       U - per-user static route

Gateway of last resort is 192.168.0.1 to network 0.0.0.0

C    1.0.0.0/8 is directly connected, Loopback0
C    192.168.0.0/24 is directly connected, Ethernet0
S*   0.0.0.0/0 [1/0] via 192.168.0.1
mycisco#

ip default-network 必須是在所到網(wǎng)絡已經(jīng)存在路由的情況下,否則執(zhí)行無效.
mycisco(config)#ip route 61.0.0.0 255.0.0.0 192.168.0.1
mycisco#show 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
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
       U - per-user static route

Gateway of last resort is not set

C    1.0.0.0/8 is directly connected, Loopback0
S    61.0.0.0/8 [1/0] via 192.168.0.1
C    192.168.0.0/24 is directly connected, Ethernet0

接著我們執(zhí)行:
mycisco(config)#ip default-network 61.0.0.0

再看路由表:
mycisco#show 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
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
       U - per-user static route

Gateway of last resort is 192.168.0.1 to network 61.0.0.0

C    1.0.0.0/8 is directly connected, Loopback0
S*   61.0.0.0/8 [1/0] via 192.168.0.1
C    192.168.0.0/24 is directly connected, Ethernet0