Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Source routing is a way of moving a packet through a network in which the path is predetermined by the source or some device that tells the source about the path. The path information is placed in the packet. When the packet arrives at a switching device, no forwarding decision is necessary. The device looks at the path information in the packet to determine the port on which it should forward the packet. This is the opposite of hop-by-hop IP routing, where packets contain only the destination address and routers at each junction in the network determine how best to forward the packet.

With source routing, the end systems are smart and the network does simple layer 2 switching. With network routing, the intelligence is in the network and the end systems do not get involved in the routing process. Both have merit. The Internet has become a success because routers take care of learning the network topology and making forwarding decisions. This simplifies host connections. On the other hand, performance can be improved by setting up routes in advance and doing "fast switching" in the network.

Source routing assumes that the source knows about the topology of the network, and can therefore specify a path. However, it is not always possible to expect end-user's systems to learn a network's topology. This gets more difficult as the network grows, and is nearly impossible on the Internet where different provider networks are joined together. From a security point of view, it is unwise to allow the sender to control the path of packets through the network.

Explicit routing is similar to source routing, except that the source gets a predefined path from a third-party device that has determined routes or had them configured by an administrator. MPLS (Multiprotocol Label Switching) supports explicit routing.

IBM Token Ring networks use a source-routing scheme that can provide network devices with information about where packets should go and how to get there. In source routing, the packets themselves hold the forwarding information. While this sounds like routing, the source-routing bridge is simply a forwarding device that knows the addresses of other bridges.

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