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

Policy-based routing, generally called "policy routing", could be the usage of route maps to determine the path a packet is going to take to go to its final destination. As you progress using your CCNP studies and continue for the CCIE (as well as to a Cisco Quality Of Service certification), you'll find that traffic may be "marked" by policy routing to be able to give different amounts of intend to various classes of traffic. (This is done by marking the traffic and placing different classes of traffic in several queues in the router, allowing the administrator to provide some traffic higher priority for transmission.)

There are some basic policy routing rules you need to know:

Policy routing doesn't modify the destination from the packet, but does customize the path that is taken up make it happen.

Policy routing can forward traffic using the source IP address or destination IP address (with the use of a long ACL).

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

Applying policy routing with 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 insurance policy globally applies the road map to packets generated about the router, this is not on all packets received on all interfaces.

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

R2#show ip policy

Interface Route map

local CHANGE_NEXT_HOP

Serial0 CHANGE_NEXT_HOP

And here's the important rule to consider....

If a packet doesn't match any from the specific criteria in a very route map, or does match a line which has an explicit deny statement, the data is sent to the routing process and you will be processed normally. If you don't wish to route packets that do not meet any route map criteria, the set command can be used to send those packets for the null0 interface. This set command medicine final set command in the route map.

There are four possibilities for an incoming packet when route maps have been in use. The following example illustrates them all.

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 path map continues to be applied towards the router's ethernet0 interface, a packet sourced from 20.1.1.1 would satisfy the first line from 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 another permit statement (sequence number 20). Since there isn't action listed, this packet would return to the routing engine to endure the traditional routing procedure. All traffic that didn't match these two addresses would be also routed normally - there would be no action taken by the road map.

Perhaps we desire to specifically block traffic sourced from 20.3.3.3 or 20.4.4.4. We can use multiple match statements in a single route map, and possess packets matching the two addresses sent on 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 will probably be shipped to null0, causing its being discarded through the router. Any traffic that didn't match any of the route map statements will be returned towards the routing engine for normal processing.

Knowing policy routing and the way to put it to use are essential skills for passing the BSCI exam, earning your CCNP, and achieving more vital in the current marketplace. Get some hands-on practice in a CCNA / CCNP home lab or rack rental to go as well as learning the thought, and you should be writing and applying policy routing right away at all.