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.^[a] This permits routing of packets originating from different sources to different networks even when the destinations are the same and can be useful when interconnecting several private networks.

Policy-based routing may also be based on the size of the packet, the protocol of the payload, or other information available in a packet header or payload.

In the Cisco IOS, PBR is implemented using route maps.^[1] Linux supports multiple routing tables since version 2.2.^[2] FreeBSD supports PBR using either IPFW, IPFilter or OpenBSD's PF.

PBR can be used to redirect traffic to a proxy server by using a PBR-aware L3-switch (router). In such deployment, specific source traffic (e.g. HTTP, FTP) can be redirected to a cache engine. This is known as virtual inline deployment.

• Policy routing Cisco Press article
• Policy based routing with Linux (ONLINE edition)
• Network overview by Rami Rosen

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