PBR – Policy Based Routing using Route map

About Policy-Based Routing

Policy-Based Routing – PBR gives you very simple way of controlling where packets will be forwarded before they enter in the destination-based routing process of the router.

It’s a technology that gives you more control over network traffic flow because you will not always want to send certain packets by the obvious shortest path. That is the job of routing protocol. If you want to send some traffic to the destination using some other path, you will need to use a method that will catch the packet as soon as they enter into router and decides where to send packets before they enter destination-based routing process. That’s Policy-Based routing all about.

From the text above you can easily determine that you specify PBR on the interface that receives the packet, not on the interface from which the packet is sent. The second option would be too late.

Other thing that can be deduced from above knowledge is that PBR – Policy Based Routing using Route map has the precedence over static routes and even over directly connected networks. That’s because the packet destination is decided before the packet even enter the routers brain. They will skip the normal routers brain and go out using the interface configured in route map.

This way PBR will allow you to configure IP precedence. It will be able for you to configure different path for particular traffic. You can send low priority traffic to normal DSL internet and priority traffic over a high-cost link. You could transfer corporate data over one fast link while sending routine non important data over another slow link.

How it works?

PBR works with:

  • Policy
  • Which is defined by Route map
  • Which is composed of Statements

PBR is configured using policies that will deny or allow specific paths by reading the identity of a particular destination system, protocol, or even the size of packets sent. Is mostly decided using extended access lists.

Packets that are received on an interface with PBR enabled are filtered by route maps. The route maps is the mean to create the policy. Or you can say, using the route map you will build the PBR policy that will then say where the packets will be forwarded.

Route maps are composed of statements that can be permit or deny:

  • You need to be careful, if there is no match criteria in the route map, the route map will be applied to all packets.
  • If the statement is marked as permit the packets meeting the match criteria will be processed by that route map.
  • If a statement is marked as deny, the packets meeting the match criteria are not processed by that route map. It is basically a way to use the match criteria to say what will not be processed with that route map.

I can suggest that you rather use permit route map statement as it is more logical to implement. The deny statement is actually used only if you need to catch most of the packet except few of them. The permit is the statement to catch and process only specific packets, but only a small part of the whole address space for example.




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