Policy routing is often a major topic on the BSCI exam, and you should find a lot of policy routing happening in today's production networks. But precisely what is policy routing?

Policy-based routing, generally called "policy routing", is the utilization of route maps to look for the path a packet will take to access its final destination. As you progress using your CCNP studies and continue on the CCIE (or a Cisco Quality Of Service certification), visitors traffic might be "marked" by policy routing as a way to give different numbers of plan to various classes of traffic. (This is done by marking the traffic and placing the several classes of traffic in different queues inside router, allowing the administrator to provide some traffic higher priority for transmission.)

There are a handful of basic policy routing rules you should know:

Policy routing doesn't get a new destination from the packet, but does modify the path that is delivered to get there.

Policy routing can forward traffic based on the source IP address or even the destination IP address (with the using a prolonged ACL).

Policy routing might be configured in the interface level, or globally.

Applying policy routing by using an interface affects only packets arriving on that interface:

R2(config)#int s0

R2(config-if)#ip policy route-map CHANGE_NEXT_HOP

Applying the protection globally applies the route map to packets generated around the router, and not on all packets received on all interfaces.

Whether you're running policy routing with the interface level, on packets created locally, or both, always run the command show ip policy to make sure you have the best route maps for the proper interfaces.

R2#show ip policy

Interface Route map

local CHANGE_NEXT_HOP

Serial0 CHANGE_NEXT_HOP

And here's the large rule to remember....

If a packet doesn't match any in the specific criteria in the route map, or does match a line which includes an explicit deny statement, the information is shipped for the routing process and are processed normally. If you don't need to route packets that will not meet any route map criteria, the set command must be used to send those packets for the null0 interface. This set command medicine final set command within the route map.

There are four possibilities to have an incoming packet when route maps will be in use. The following example illustrates these.

R2(config)#access-list 29 permit host 20.1.1.1

R2(config)#access-list 30 permit host 20.2.2.2

R2(config)#access-list 31 permit host 20.3.3.3

R2(config)#access-list 32 permit host 20.4.4.4

R2(config)#route-map EXAMPLE permit 10

R2(config-route-map)#match ip address 29

R2(config-route-map)#set ip next-hop 40.1.1.1

R2(config-route-map)#route-map EXAMPLE permit 20

R2(config-route-map)#match ip address 30

Assuming the road map has been applied to the router's ethernet0 interface, a packet sourced from 20.1.1.1 would fulfill the first line with the route map and have its next-hop IP address set to 40.1.1.1.

A packet sourced from 20.2.2.2 would match the next permit statement (sequence number 20). Since there isn't any action listed, this packet would return for the routing engine to endure the normal routing procedure. All traffic that didn't match these two addresses would even be routed normally - there would be no action taken by the road map.

Perhaps we want to specifically block traffic sourced from 20.3.3.3 or 20.4.4.4. We can use multiple match statements in one single route map, and have packets matching the above addresses sent towards the bit bucket - the interface null0.

R2(config)#route-map EXAMPLE permit 30

R2(config-route-map)#match ip address 31

R2(config-route-map)#match ip address 32

R2(config-route-map)#set ?

as-path Prepend string for the BGP AS-path attribute

automatic-tag Automatically compute TAG value

comm-list set BGP community list (for deletion)

community BGP community attribute

dampening Set BGP route flap dampening parameters

default Set default information

extcommunity BGP extended community attribute

interface Output interface

ip IP specific information

level Where to import route

local-preference BGP local preference path attribute

metric Metric value for destination routing protocol

metric-type Type of metric for destination routing protocol

origin BGP origin code

tag Tag value for destination routing protocol

weight BGP weight for routing table

R2(config-route-map)#set interface null0

Any traffic matching ACLs 31 or 32 is going to be delivered to null0, causing its being discarded by the router. Any traffic that didn't match any from the route map statements is going to be returned for the routing engine for normal processing.

Knowing policy routing and the way to put it on are crucial skills for passing the BSCI exam, earning your CCNP, and having more vital in our employment market. Get some hands-on practice in the CCNA / CCNP home lab or rack rental to go along with learning the idea, and you will be writing and applying policy routing right away at all.