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Policy routing can be a major topic on the BSCI exam, and you should find a large amount of policy routing occurring in our production networks. But precisely what is policy routing?

Policy-based routing, generally referred to as "policy routing", will be the using route maps to discover the path a packet will need to access its final destination. As you progress through your CCNP studies and embark on to the CCIE (or a Cisco Quality Of Service certification), you will find that traffic might be "marked" by policy routing as a way to give different levels of want to various classes of traffic. (This is done by marking the traffic and placing the several classes of traffic in several queues inside router, allowing the administrator to present some traffic higher priority for transmission.)

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

Policy routing doesn't affect the destination from the packet, but does get a new path that is taken to arrive.

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

Policy routing may be configured on 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 protection globally applies the path map to packets generated about the router, not on all packets received on all interfaces.

Whether you're running policy routing on the interface level, on packets created locally, or both, always run the command show ip policy to be sure you've got the right route maps around the proper interfaces.

R2#show ip policy

Interface Route map

local CHANGE_NEXT_HOP

Serial0 CHANGE_NEXT_HOP

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

If a packet doesn't match any of the specific criteria inside a 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 want to route packets that will not meet any route map criteria, the set command can be used to send those packets towards the null0 interface. This set command should be the final set command inside route map.

There are four possibilities on an incoming packet when route maps come 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 on the router's ethernet0 interface, a packet sourced from 20.1.1.1 would satisfy the first line from the route map and possess 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 towards the routing engine to pass through the standard routing procedure. All traffic that did not match these addresses would also 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 a single route map, and have packets matching the two 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 to get a 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 provided for null0, producing its being discarded from the router. Any traffic that didn't match any with the route map statements will likely be returned on the routing engine for normal processing.

Knowing policy routing and how to put it on are crucial skills for passing the BSCI exam, earning your CCNP, and having more significant in the current marketplace. Get some hands-on practice in a CCNA / CCNP home lab or rack rental to go in addition to learning the idea, and you will probably be writing and applying policy routing in no time in any respect.