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Cut-Through Routing

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Routers receive packets from one or more inputs and make forwarding decisions about where to send the packets. Traditional software routers (as opposed to routing switches) read each packet header and examine the fields to determine how the packet should be forwarded. This takes time and affects performance. Add multiple router hops into the path between source and destination, and the requirement to also filter packets, perform accounting, and encrypt data, and you come up with a low-performance network link. Some routers use caching techniques that detect a flow of packets and store the routing information of the first packet in memory. Subsequent routing decisions are made from the information in memory, thus boosting performance somewhat.

Cut-through routing (sometimes called shortcut routing) is an approach in which devices on different subnets can directly communicate with one another without going through a router. The approach is used on switching fabrics such as ATM networks in which multiple LISs (logical IP subnets) overlay the ATM switching fabric. While routing is normally required between LISs, an end system in one LIS can connect directly with an end system in another LIS by using a cut-through route. The cut-through route is created between the end system as a virtual circuit across the ATM switching fabric.

Typically, the first few packets are initially routed, but if a long flow is detected, the ATM address of the destination is obtained by the source, which then sets up a virtual connection across the ATM fabric directly to the destination, switching all subsequent packets and bypassing the routers. This process is often referred to as "route once, switch many." The related entries, next, provide further information.

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