二、需求概述：
R1、R2作为本地网络12.0.0.0/24、21.0.0.0/24的网关，为连接远端R5上的网段5.0.0.0/24，分别向两个ISP（ISP1、ISP2）申请了1条Internet线路。R3、R4分别给R1、R2一个独立的公网地址（R3分配R1：13.0.0.1、分配R2：13.0.0.2；R4分配R1：14.0.0.1、分配R2：24.0.0.2）。由于R1、R2为Stub AS，故考虑使用浮动静态路由 负载均衡的方式对外访问。其中R1主路由下一跳指向R3 e0/0；R2主路由下一跳指向R4 e0/1。
三、实现方法：
双ISP双线接入时，网关所使用的nat内部全局地址应为所选线路对应的IP地址（或者该端口所对应的pool）。由于不同ISP有各自对应的出端口，NAT为使用不同的ISP地址段，内部本地地址池除了要判断本地Vlan的地址外，还要判断所选路由对应出接口。所以这里要使用到route-map针对源地址和出端口进行筛选。而判断出端口的任务，就交由带track的浮动静态路由实现。
四、配置过程：
R1:
track 3 ip sla 3
track 4 ip sla 4
track 50 ip route 5.0.0.0 255.255.255.0 reachability
interface e0/2
ip address 13.0.0.1 255.255.255.0
ip nat outside
interface e0/3
ip address 14.0.0.1 255.255.255.0
ip nat outside
interface e0/0
ip address 12.0.0.1 255.255.255.0
ip nat inside
standby 12 ip 12.0.0.254
standby 12 priority 150
standby 12 preempt
standby 12 track 50 decrement 100
interface e0/1
ip address 21.0.0.1 255.255.255.0
ip nat inside
standby 21 ip 21.0.0.254
standby 21 preempt
standby 21 track 50 decrement 100
ip nat inside source route-map TO_R3_NAT interface e0/2 overload
ip nat inside source route-map TO_R4_NAT interface e0/3 overload
ip route 5.0.0.0 255.255.255.0 13.0.0.3 50 track 3
ip route 5.0.0.0 255.255.255.0 14.0.0.4 100 track 4
ip access-list standard VLAN_12
permit 12.0.0.0 0.0.0.255
ip access-list standard VLAN_21
permit 21.0.0.0 0.0.0.255
ip sla 3
icmp-echo 13.0.0.3
exit
ip sla schedule 3 life forever start-time now
ip sla 4
icmp-echo 14.0.0.4
exit
ip sla schedule 4 life forever start-time now
route-map TO_R3_NAT permit 10
match ip address VLAN_12 VLAN_21
match interface e0/2
route-map TO_R4_NAT permit 10
match ip address VLAN_12 VLAN_21
match interface e0/3
R2：
track 3 ip sla 3
track 4 ip sla 4
track 50 ip route 5.0.0.0 255.255.255.0 reachability
interface Loopback 0
ip address 2.2.2.2 255.255.255.255
interface e0/2
ip address 23.0.0.2 255.255.255.0
ip nat outside
no shut
interface e0/3
ip address 24.0.0.2 255.255.255.0
ip nat outside
no shut
interface e0/1
ip address 12.0.0.2 255.255.255.0
ip nat inside
standby 12 preempt
standby 12 ip 12.0.0.254
standby 12 track 50 decrement 100
no shut
interface e0/0
ip address 21.0.0.2 255.255.255.0
ip nat inside
standby 21 ip 21.0.0.254
standby 21 priority 150
standby 21 preempt
standby 21 track 50 decrement 100
no shut
ip nat inside source route-map TO_R3_NAT interface e0/2 overload
ip nat inside source route-map TO_R4_NAT interface e0/3 overload
ip route 5.0.0.0 255.255.255.0 23.0.0.3 100 track 3
ip route 5.0.0.0 255.255.255.0 24.0.0.4 50 track 4
ip access-list standard VLAN_12
permit 12.0.0.0 0.0.0.255
exit
ip access-list standard VLAN_21
permit 21.0.0.0 0.0.0.255
ip sla 3
icmp-echo 23.0.0.3
exit
ip sla schedule 3 life forever start-time now
ip sla 4
icmp-echo 24.0.0.4
exit
ip sla schedule 4 life forever start-time now
route-map TO_R3_NAT permit 10
match ip address VLAN_12 VLAN_21
match interface e0/2
route-map TO_R4_NAT permit 10
match ip address VLAN_12 VLAN_21
match interface e0/3
R3:
interface Loopback 0
ip address 3.3.3.3 255.255.255.255
interface e0/0
ip address 13.0.0.3 255.255.255.0
no shut
interface e0/1
ip address 23.0.0.3 255.255.255.0
no shut
interface S1/0
ip address 35.0.0.3 255.255.255.0
encapsulation ppp
no shut
exit
router eigrp 12345
passive-interface e0/0
passive-interface e0/1
passive-interface Loopback 0
network 3.3.3.3 0.0.0.0
network 13.0.0.3 0.0.0.0
network 23.0.0.3 0.0.0.0
network 35.0.0.3 0.0.0.0
eigrp router-id 3.3.3.3
R4:
interface Loopback 0
ip address 4.4.4.4 255.255.255.255
interface e0/0
ip address 14.0.0.4 255.255.255.0
no shut
interface e0/1
ip address 24.0.0.4 255.255.255.0
no shut
interface S1/0
ip address 45.0.0.4 255.255.255.0
encapsulation ppp
no shut
exit
router eigrp 12345
passive-interface e0/0
passive-interface e0/1
passive-interface Loopback 0
network 4.4.4.4 0.0.0.0
network 14.0.0.4 0.0.0.0
network 24.0.0.4 0.0.0.0
network 45.0.0.4 0.0.0.0
eigrp router-id 4.4.4.4
R5:
interface Loopback 0
ip address 5.5.5.5 255.255.255.255
interface Loopback 1
ip address 5.0.0.1 255.255.255.0
interface Serial1/0
ip address 35.0.0.5 255.255.255.0
encapsulation ppp
no shut
interface Serial1/1
ip address 45.0.0.5 255.255.255.0
encapsulation ppp
no shut
exit
router eigrp 12345
passive-interface Loopback 0
passive-interface Loopback 1
network 5.0.0.1 0.0.0.0
network 5.5.5.5 0.0.0.0
network 35.0.0.5 0.0.0.0
network 45.0.0.5 0.0.0.0
eigrp router-id 5.5.5.5
五、结果测试：
本实验假设网关R1使用R3作为主路由，R2使用R4作为主路由。由于R1使用到SLA监控R3端口IP的可达性，因此先查看SLA状态：
要在静态路由中使用sla的状态，必须先用track跟踪sla状态：
最后，检查R1的浮动静态路由是否正确使用到SLA返回的状态选择路由：
经过配置，在模拟互联网中不存在12.0.0.0/24和21.0.0.0/24两个内网IP地址段的情况下，客户端可正常连接到远端网段：
pc1：
由于R1使用ISP1（R3）作为主路由，因此经过R1的内网数据包被R1的NAT进程映射到R3所分配的IP地址。
R1 NAT状态debug：
重头戏来了！必须验证R1能够在R3实效的情况下正确切换到R4。现在关闭R3的e0/0端口，并检查SLA3的返回情况：
同样，track3的状态随之改变：
我们最为关心的路由表情况：
当然，测试连通性是最主要的：
其实，由于R4没有R1-R3的路由，NAT肯定是以R1-R4的端口IP作转换的。以防万一，检查R1的NAT转换情况：
至此实验完成！
