Policy-based routing

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In computer networking, policy-based routing (PBR) is a technique used to make routing decisions based on policies set by the network administrator.

When a router receives a packet it normally decides where to forward it based on the destination address in the packet, which is then used to look up an entry in a routing table. However, in some cases, there may be a need to forward the packet based on other criteria. For example, a network administrator might want to forward a packet based on the source address, not the destination address. This should not be confused with source routing.

Policy-based routing may also be based on the size of the packet, the protocol of the payload, or some other characteristic. It can also be based on a combination of characteristics. Policy-based routing can be based on the source and the destination, that is, when the packet comes from this source and wants to go to that destination then route to this next hop or onto that specific interface. This permits routing over different links or towards different networks even while the destination is the same but depending on where the packet is originating from. This can be useful when interconnecting several private networks. If you have the possibility to design an architecture without IP overlaps, then that is highly recommended over using PBR.

In the Cisco IOS, PBR is implemented using route maps.^[1].

Linux supports multiple routing tables since version 2.2.^[2]