PBR : Policy Based Routing

Policy-Based Routing (PBR) allows you to use ACLs and route maps to selectively modify and route IP packets in hardware.

(PBR) allows you to use ACLs and route maps to selectively modify and route IP packets in hardware. Basically, the ACLs classify the traffic and route maps that match on the ACLs set routing attributes for the traffic.

A PBR policy specifies the next hop for traffic that matches the policy:

·         For standard ACLs with PBR, you can route IP packets based on their source IP address.

·         For extended ACLs with PBR, you can route IP packets based on all of the matching criteria in the extended ACL.

The problem that many network engineers find with typical routing systems and protocols is that they are based on routing the traffic based on the destination of the traffic. Now under normal situations this is fine, but when the traffic on your network requires a more hands on solution policy based routing takes over.

Destination based routing systems make it quite hard to change the routing behavior of specific traffic. With PBR, a network engineer has the ability to dictate the routing behavior based on a number of different criteria other than destination network, including source or destination network, source or destination address, source or destination port, protocol, packet size, and packet classification among others.

PBR also has the ability to implement QoS by classifying and marking traffic at the network edge and then using PBR throughout the network to route marked traffic along a specific path.

So why would you do this? Well consider a company that has two links between locations, one a high bandwidth, low delay expensive link and the other a low bandwidth, higher delay lower expense link.

Now using traditional routing protocols the higher bandwidth link would get most if not all of the traffic sent across it based on the metric savings obtained by the bandwidth and/or delay (using EIGRP or OSPF) characteristics of the link. PBR would give you the ability to route higher priority traffic over the high bandwidth/low delay link while sending all other traffic over the low bandwidth/high delay link.

This way the traffic which requires the characteristics of the high bandwidth/low delay link would be possible without sending all traffic over the link.

http://blog.pluralsight.com/pbr-policy-based-routing

 http://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfpbr.html