Hi,
1. You need to create ACL that will match internal LAN subnet.
2. Configure 3 default static route pointing ISPs IP address.
3. Create route-maps that will match ACL above.
And Cisco CEF will take care of load-balancinf between these links.
example:
ip cef
interface FastEthernet0/0
Description Primary link ISP1
ip address 12.x.x.x 255.255.255.240
ip nat outside
interface FastEthernet1/0
Description Secondary link ISP2
ip address 76.x.x.x. 255.255.255.252
ip nat outside
interface FastEthernet0/1
Description link ISP3
ip address 96.x.x.x. 255.255.255.252
ip nat outside
interface FastEthernet1/1
Description Inside LAN segment
ip address 172.16.20.1 255.255.255.0
ip nat inside
access-list 100 permit ip 172.16.20.0 0.0.0.255 any
route-map isp1 permit 10
match ip address 100
match interface FastEthernet0/0
route-map isp2 permit 10
match ip address 100
match interface FastEthernet1/0
route-map isp3 permit 10
match ip address 100
match interface FastEthernet0/1
ip nat inside source route-map isp1 interface FastEthernet0/0 overload
ip nat inside source route-map isp2 interface FastEthernet1/0 overload
ip nat inside source route-map isp3 interface FastEthernet0/1 overload
ip route 0.0.0.0 0.0.0.0 12.y.y.y track 1
ip route 0.0.0.0 0.0.0.0 76.y.y.y track 2
ip route 0.0.0.0 0.0.0.0 96.y.y.y track 3
Also you could add IP SLA to monitor those links. In case of failure one of the links it will be removed from routing table.
ip sla 1
icmp-echo 4.4.4.4 source-interface FastEthernet0/0
timeout 1000
ip sla schedule 1 life forever start-time now
ip sla 2
icmp-echo 4.4.4.4 source-interface FastEthernet1/0
timeout 1000
ip sla schedule 2 life forever start-time now
ip sla 3
icmp-echo 4.4.4.4 source-interface FastEthernet0/1
timeout 1000
ip sla schedule 3 life forever start-time now
track 1 ip sla 1 reachability
track 2 ip sla 2 reachability
track 3 ip sla 3 reachability
http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/813-cisco-router-ipsla-basic.html
Hope it will help.
Best regards,
Abzal
Best regards,
Abzal
