Normally, routers forward traffic based only on the final destination address, but there are times when you want the router to make a forwarding decision based on the source address, the type of traffic, or some other criteria. These types of forwarding decisions, based on some criteria or policy the system administrator has configured, are called policy-based routing.
A router can be configured to make a forwarding decision based on several things,
including
· Source address
· Source/destination address pair
· Destination address
· IP packet type (TCP, UDP, ICMP, and so on)
· Service type (Telnet, FTP, SMTP)
· Precedence bits in the IP header
Typically, configuring policy-based routing consists of the following three steps:
1. Build a filter to separate the traffic that needs a specific policy applied from
the normal traffic.
2. Build a policy.
3. Implement the policy.
On a Cisco router, a policy is built using route maps and is implemented with
interface commands.
Lets have a look at one such example -
In the figure below, we wish to send the HTTP traffic over the frame-relay link whereas rest all traffic should flow via satelite link. So, the configuration for this would be -
 |
| Example - Policy Based Routing |
Step 1 - Build a filter using access list
access-list 150 permit tcp any eq http any
access-list 150 permit tcp any any eq http
Step 2 - Creat a policy
route-map http-PBR permit 10
match ip address 150
set ip next-hop 192.168.10.1
Step 3 - Apply the policy to the interface
interface ethernet 0
ip policy route-map http-PBR
So, Amigos I am sure this 1-2-3 mantra will help you in creating policy maps easily.