学习-ip default-network在EIGRP中的应用，由图可以看出R2与R3之间的网络分割内网与外网。R2是一个边界路由器，R1为内部路由器，所以在R1没有必要知道，网络中所有路由信息，只要知道，我通过那个接口可以到外网去就可以了，在EIGRP中，我们可以通过ip default-network 命令实现，具体如下：
-------------------------------------------------------------------------------------------------------
在R2 上没有配置network 23.0.0.0的时候，R1的路由表
R1#show ip ro
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
并没有收到带*的路由条目，这就是RIP与EIGRP的不同，因为ip default-network
在EIGRP中需要路由更新路由包发送带*的路由。
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
112.0.0.0/24 is subnetted, 1 subnets
C 112.1.1.0 is directly connected, FastEthernet0/0
R2的路由表为
R2#show ip ro
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 23.1.1.3 to network 0.0.0.0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
* 23.0.0.0/24 is subnetted, 1 subnets //由ip default-network产生
C 23.1.1.0 is directly connected, Serial1/1
112.0.0.0/24 is subnetted, 1 subnets
C 112.1.1.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 23.1.1.3
-------------------------------------------------------------------------------------------------------
配置network 23.0.0.0了后R1路由表为
R1#show ip ro
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 112.1.1.2 to network 23.0.0.0
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
D* 23.0.0.0/8 [90/2172416] via 112.1.1.2, 00:27:11, FastEthernet0/0 存在带*的了吧
112.0.0.0/24 is subnetted, 1 subnets
C 112.1.1.0 is directly connected, FastEthernet0/0
配置后的R2路由表为
R2#show ip ro
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 23.1.1.3 to network 0.0.0.0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
* 23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 23.1.1.0/24 is directly connected, Serial1/1
D* 23.0.0.0/8 is a summary, 00:28:39, Null0 //由network 23.0.0.0产生，这里有个地方一定要注意，就是ip default-network 网络主网号 不然就出现后面的问题，请看注
112.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 112.1.1.0/24 is directly connected, FastEthernet0/0
D 112.0.0.0/8 is a summary, 00:28:39, Null0
S* 0.0.0.0/0 [1/0] via 23.1.1.3
R1#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/96/180 ms
注：ip default network 网络主网号 如果不是，这就会出现
R2#sh ip rou
Gateway of last resort is 23.1.1.3 to network 0.0.0.0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
* 23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 23.1.1.0/24 is directly connected, Serial1/1
S* 23.0.0.0/8 [1/0] via 23.1.1.0 它不是指向null 0
12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 12.1.1.0/24 is directly connected, Serial1/0
D 12.0.0.0/8 is a summary, 00:02:24, Null0
S* 0.0.0.0/0 [1/0] via 23.1.1.3
分析：S* 23.0.0.0/8 [1/0] via 23.1.1.0
它不是指向null 0，而是指定我们在ip default-network 23.1.1.0.如果这样的话，就出错了，我们在PING 3.3.3.3时
R1#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
为什么会不通呢
1.首先：让我们分板数据包正常到R3的路由路径：（1）R1查找到3.3.3.3的路由，确是没有找到完全匹配3.3.3.3，只有一条D* 23.0.0.0/8 [90/2172416] via 112.1.1.2, 00:27:11, FastEthernet0/0通过EIGRP学过来路认路由 （2）数据包到达R2上，这时也没有查到匹配3.3.3.3的条目，这是S* 23.0.0.0/8 [1/0] via unll 0
S* 0.0.0.0/0 [1/0] via 23.1.1.3有两条默认路由，我们选那条呢？以前讲到了，ip defalut-network 在有路由协议时，才有作用。所以现在匹配，S* 0.0.0.0/0 [1/0] via 23.1.1.3 将数据包成功地发到了R3上。（3）在R3上，通过S* 0.0.0.0/0 [1/0] via 23.1.1.2发到R2 R2看到12.1.1.1是它直连的，R1收到了自己发出去的数据包。OK 完成路由的过程。
2。再讲不通的情况 看一个配置文件吧
R2#sh run
ip classless
ip default-network 23.1.1.0
ip route 0.0.0.0 0.0.0.0 23.1.1 .3
ip route 23.0.0.0 255.0.0.0 23.1.1.0 将数据出口指向了23.1.1.0
no ip http server
no ip http secure-server
虽然在R1看到的D* 23.0.0.0/8 [90/2681856] via 12.1.1.2, 00:07:52, Serial1/0这条信息，但ip deflaut-network 23.1.1.0 所以数据包跟本发不出去。在R1就被丢了，可以从debug ip packet信息中看出
*Apr 8 21:59:37.255: IP: tableid=0, s=12.1.1.1 (local), d=3.3.3.3 (Serial1/0), routed via FIB
*Apr 8 21:59:37.259: IP: s=12.1.1.1 (local), d=3.3.3.3 (Serial1/0), len 100, sending。
