NAT failover with DUAL ISP on a router with Policy Based Routing

INTRODUCTION:-

WAN Failover and Load Balancing allows you to designate the Router interface or one of the user-assigned interfaces as a Secondary or backup WAN port. The  secondary WAN port can be used in a simple active/passive setup, where traffic is only routed through the secondary WAN port if the primary WAN port is down and/or unavailable.

This is an generic example of how to configure NAT when there are multiple ISP’s for internet connectivity and we want proper Failover we can say that when Primary ISP goes down then Secondary takes over with correct NAT happening using the secondary ISP’s public ip address.  For dia-gram please go through this url http://www.2shared.com/photo/FYF4kN6X/DRAW.html.

Concept:
Small business with a single router (i.e Cisco 1900/2800/2900 ISR), has two ISPs, and would like to use one ISP as the main provider, and the second ISP as a backup in case the first one fails.

Configuration

Here i am going to post only Home Router confiugation which is used in Lab invironment as HOME.RTR for detail click on mentioned url.

1) Configure each WAN interface to connect to each ISP, respectively:

interface FastEthernet0/0
description Primary-Wan
ip address 101.1.1.1 255.255.255.252
ip nat outside

interface FastEthernet0/1
description backup-Wan
ip address 102.1.1.1 255.255.255.252
ip nat outside

2) Create the Route Maps that will be used for NATting traffic:

route-map backup permit 20
match ip address internet
match interface FastEthernet0/1
route-map primary permit 10
match ip address internet
match interface FastEthernet0/0

3) Configure Lan interface

interface FastEthernet1/0
ip address 192.168.1.1 255.255.255.0
ip nat inside

Note: ACL internet allows traffic from the LAN.

ip access-list extended internet
permit ip any any

4) Configure the two NAT statements required so that either interface can provide NATting, out each respective WAN interface:

ip nat inside source route-map backup interface FastEthernet0/1 overload
ip nat inside source route-map primary interface FastEthernet0/0 overload

5) Update your static routes:

ip route 0.0.0.0 0.0.0.0 101.1.1.2 10
ip route 0.0.0.0 0.0.0.0 102.1.1.2 20

6) Create an EEM Script for automatically clear ip nat translatoin :

Introduction Embedded Event Manager

Cisco IOS Embedded Event Manager (EEM) is a powerful tool integrated with Cisco IOS Software for system management from within the device itself. EEM offers the ability to monitor events and take informational, corrective, or any desired action when the monitored events occur or when a threshold is reached. Capturing the state of the router during such situations can be invaluable in taking immediate recovery actions and gathering information to perform root-cause analysis. Network availability is also improved if automatic recovery actions are performed without the need to fully reboot the routing device.

For example if we talk about our topology there are two ISPs which configured on router as f0/0 and f0/1 these are natted suppose traffic is going through f0/0 (primary) if it becomes down or unavialable so our router will take much time to switch over to another interface f0/1 (secondary) to avoid this we have to configure EEM and have to specify when interface will goes down this EEM script will automatically run a command “clear ip nat translation *”

Commands will be

event manager applet CLEARNAT1
event syslog pattern “LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down”
action 1.0 cli command “enable”
action 2.0 cli command “clear ip nat translation *”
event manager applet CLEARNAT2
event syslog pattern “LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down”
action 1.0 cli command “enable”
action 2.0 cli command “clear ip nat translation *”