Hello Kamaradz17,
standard routing is destination based routing: the router looks at the destination address of the packet to find out the outgoing interface and IP next-hop (when it applies) by looking at the IP routing table.
The most specific prefix is used first, if there is a tie administrative distance decides which route to use.
There are some scenarios where you would like to route based on type of traffic or source address or other criteria that are beyond destination based routing.
When destination based routing is not enough to achieve the desired result Policy Based Routing is a valuable tool: PBR uses a route-map to define the match criteria and the actions (set commands) to be performed on matching packets.
A typical set action is to set an IP next-hop that uses a path that standard routing would not use because it has an higher IGP metric.
Match criteria can use an extended ACL to match source address, destination address, protocol and L4 port (if applicable for TCP or UDP only).
Thanks to this PBR is a tool that allow to divert to move specific traffic flows from the shortest path found from IGP to an alternate path that is likely less loaded then primary path.
Not using PBR just means that you use standard destination based routing and the packets are routed normally.
To be noted PBR when used is applied to the interface that receives the traffic to be diverted, not to the outgoing interface to the primary path chosen by the routing protocol in use.
So PBR has special uses for specific scenarios.
Example:
make VOIP traffic to travel on secondary path to be separated from data traffic.
Most of the times PBR if configured has to be configured at both ends of the secondary path to allow for symmetric routing of specified traffic flows.
Hope to help
Giuseppe
