实验目的：使用偏移列表offset-list修改metric值。使R3在正常情况下到网络1.0.0.0/8走Serial2/1，只有当使用这跳串形链路失败时，才使用这条快速以太链路。只想把这条快速以太链路作为备份链路
基本配置
R1#show running-config
interface Loopback0
ip address 1.1.1.1 255.255.255.0
interface FastEthernet0/0
ip address 13.0.0.1 255.255.255.0
interface Serial2/1
ip address 12.0.0.1 255.255.255.0
router rip
network 12.0.0.0
network 13.0.0.0
network 1.0.0.0
R2#show running-config
interface Serial2/1
ip address 12.0.0.2 255.255.255.0
interface Serial2/2
ip address 23.0.0.2 255.255.255.0
router rip
network 12.0.0.0
network 23.0.0.0
R3#show running-config
interface FastEthernet0/0
ip address 13.0.0.3 255.255.255.0
interface Serial2/1
ip address 23.0.0.3 255.255.255.0
router rip
network 13.0.0.0
network 23.0.0.0
------------------------------------------------------------------------------\
R3(config-router)# do sh ip rou
Gateway of last resort is not set
R 1.0.0.0/8 [120/1] via 13.0.0.1, 00:00:02, FastEthernet0/0
23.0.0.0/24 is subnetted, 1 subnets
C 23.0.0.0 is directly connected, Serial2/1
R 12.0.0.0/8 [120/1] via 23.0.0.2, 00:00:12, Serial2/1
[120/1] via 13.0.0.1, 00:00:02, FastEthernet0/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, FastEthernet0/0
查看R3路由表，看到有1条到10.0.0/8 的路由,etric 1.
R3(config-router)#int f0/0
R3(config-if)#shut
R3(config-if)#do sh ip rou
Gateway of last resort is not set
R 1.0.0.0/8 [120/2] via 23.0.0.2, 00:00:06, Serial2/1
23.0.0.0/24 is subnetted, 1 subnets
C 23.0.0.0 is directly connected, Serial2/1
R 12.0.0.0/8 [120/1] via 23.0.0.2, 00:00:06, Serial2/1
R 13.0.0.0/8 [120/2] via 23.0.0.2, 00:00:06, Serial2/1
关闭f0/0接口后，此时路由表中到1.0.0./8的路由下一跳是R2。Metric值是2。
在正常情况下。依照rip 的特性，rip会选择跳数少的路径。在这不考滤链路带宽。
假设此题只想把这条快速以太链路作为备份链路，只有当经过R2失败时，才使用这条快速以太链路。
使用偏移列表，将R3经过F0/0到达1.0.0.0/8网络的Metric设为3跳。
R3(config)#access-list 1 permit 1.0.0.0 0.0.0.0
R3(config)#router rip
R3(config-router)#offset-list 1 in 3 f0/0
R3(config-if)#int f0/0
R3(config-if)#no shut
R3#debug ip rip
RIP protocol debugging is on
R3#
00:55:21: RIP: received v1 update from 13.0.0.1 on FastEthernet0/0
00:55:21: 1.0.0.0 in 4 hops
00:55:21: 12.0.0.0 in 1 hops
R3#
00:55:27: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (13.0.0.3)
00:55:27: RIP: build update entries
00:55:27: network 1.0.0.0 metric 3
00:55:27: network 23.0.0.0 metric 1
00:55:27: RIP: sending v1 update to 255.255.255.255 via Serial2/1 (23.0.0.3)
00:55:27: RIP: build update entries
00:55:27: network 13.0.0.0 metric 1
R3#
00:55:40: RIP: received v1 update from 23.0.0.2 on Serial2/1
00:55:40: 1.0.0.0 in 2 hops
00:55:40: 12.0.0.0 in 1 hops
通过调试信息看到 R3接收的1.0.0.0的更新，从F0/0通告来的metric是3。
从S2/1通告来的metric是2。 说明偏移列表在起作用。
关闭s2/1接口，验证到网络1.0.0.0，在经过R2失败时，使用f0/0这条链路。
R3(config)#int s2/1
R3(config-if)#shut
R3#sh ip rou
Gateway of last resort is not set
R 1.0.0.0/8 [120/4] via 13.0.0.1, 00:00:08, FastEthernet0/0
R 12.0.0.0/8 [120/1] via 13.0.0.1, 00:00:08, FastEthernet0/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, FastEthernet0/0
实验成功。此时到网络1.0.0.0/8下一跳R1 。Metric是4 。
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