ＯＳＰＦ ｃｏｓｔ修改
需求：
让R1到达3.3.3.0网络优选R2到达
配置：
R1
interface Loopback0
ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 13.1.1.1 255.255.255.0
ip ospf network broadcast
duplex half
!
interface Serial1/0
ip address 12.1.1.1 255.255.255.0
ip ospf network broadcast
serial restart-delay 0
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.1 0.0.0.0 area 0
network 12.1.1.1 0.0.0.0 area 0
network 13.1.1.1 0.0.0.0 area
line con 0
exec-timeout 0 0
password console
logging synchronous
login
line aux 0
line vty 0 4
R2
interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex half
!
interface Serial1/0
ip address 12.1.1.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 23.1.1.1 255.255.255.0
serial restart-delay 0
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
network 2.2.2.2 0.0.0.0 area 0
network 12.1.1.1 0.0.0.0 area 0
network 12.1.1.2 0.0.0.0 area 0
network 23.1.1.1 0.0.0.0 area 0
R3
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
ip address 13.1.1.2 255.255.255.0
duplex half
!
interface Serial1/0
ip address 23.1.1.2 255.255.255.0
serial restart-delay 0
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
network 3.3.3.3 0.0.0.0 area 0
network 13.1.1.1 0.0.0.0 area 0
network 13.1.1.2 0.0.0.0 area 0
network 23.1.1.2 0.0.0.0 area 0
验证配置;
R1#show ip 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
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 12.1.1.2, 00:36:26, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/2] via 13.1.1.2, 00:36:26, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
O 23.1.1.0 [110/65] via 13.1.1.2, 00:36:26, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, FastEthernet0/0
R1选择以太网接口直接到达R3的3.3.3.0网络，因为这条路径的COST(度量值)最低，如果选择R2到达R3,COST是129（64+64+1）
那为什么以太网接口的连得路径度量值是2。是这样的吗（1+1），环回接口的链路度量值是1
修改R1和R2接口的OSPF度量值
R1(config)#int s1/0
R1(config-if)#ip ospf cost 1
R2(config)#int s1/1
R2(config-if)#ip ospf cost 1
再次查看R1路由表
R1#show ip 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
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 12.1.1.2, 00:00:07, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/66] via 12.1.1.2, 00:00:07, Serial1/0
23.0.0.0/24 is subnetted, 1 subnets
O 23.1.1.0 [110/65] via 12.1.1.2, 00:00:07, Serial1/0
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.1.1.0 is directly connected, FastEthernet0/0
这一步中因为度量值两条路径是一样的都是2—实验测试中（我也纳闷），路由表中任然是显示以太网接口的路径，后来我就改变R1和R3以太网接口
的度量值，结果才显示路径选取走R2到达3.3.3.0网段。但是这个度量值为什么是66，我又闷啊啊？？？？？
R1#traceroute 3.3.3.3
Type escape sequence to abort.
Tracing the route to 3.3.3.3
1 12.1.1.2 20 msec 64 msec 80 msec
2 23.1.1.2 92 msec 96 msec *
总结与思考:
本实验收获：
1.ospf中不能宣告空接口，如：
network 12.1.1.1 0.0.0.0 area 0
network 12.1.1.2 0.0.0.0 area 0
我根本就没有配置12.1.1.1 的接口，这样宣告不会找到邻居的当然邻居路由表中是不会有到你的ospf路由表，本实验中因为R2和R3这样宣告空接口，致使邻居R1上没有ospf的路由表
2.为什么各个路由器的Id的Ip地址非直连的都是以32位的显示在路由表中，而我的掩码是24位
Router-id是以IPv4的形式定义的 本地有效，环回口不管你怎样配置 都是以32位显示在路由表里的
3，当修改后不起作用时，可更改以前路径上的路由器接口的度量值，如上，但是这个度量值的计算不明白？？
4。ospfCOST修改的三种方法：a.修改接口带宽，b,接口下IP ospf cost (1-65535)命令,c.参考带宽 auto-cost reference-bandwidth ref-bw(以Mbits为单位的参考带宽，取值为1-4294967，默认为100)