In order to apply QOS policies in your network, traffic should be classified first. There are many techniques available for packet classification. PBR is one of these techniques as discussed below.
PBR can be used to match a specific flow and then apply QOS policies on it, like marking this traffic or routing the traffic to special optimized or dedicated connections to handle this traffic type.
For example you may have two internet connections, one for normal internet traffic [FTP, HTTP, downloads] and another one for critical applications or to be dedicated only for use by the managers or critical departments.
In short PBR can be used to classify traffic based on extended ACLs, set IP precedence value or route traffic to specific traffic engineered connections.
Route maps can be used as follows:
Route-map QOS permit Match [ ip address | length ] Set [ip precedence | ip next-hop | set interface]
In the match statement you can match any standard or extended access-list, or even match packets with specific or ranged length.
Using extended access-lists you can match specific flows, for example you can match only email flows sourced from customer service departments to give them some priority over normal emails.
The set statements can be used to set the IP precedence value to mark packets for use by other QOS tools or within your ISP’s core. Set interface and next-hop statements can be used to override normal forwarding for this specific flow.
Hope that was useful for all of you