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This is a review of Cisco router switching modes. It is recommended that a review of Cisco’s router product line and a detailed understanding of the various router platforms such as small, medium, and high-end routers be completed prior to any examination.

At the onset, the Interface Process Buffer compares Checksum and performs CRC [Cyclic Redundancy Check] for errors. If the first packet is error-free, it is then sent to the Switch Processor. A check is made against other switching caches - Autonomous or Silicon for an existing entry. If an entry exists, then Fast Switching is used. In the case of Fast Switching, the Switch Processor is only needed to build a route table for the first packet and NOT needed afterward.

The packet is compared with the route table and a route or exit Interface Buffer port is assigned. Subsequent following packets are switched (routed) to the same destination. This is called Fast Switching as an entry already exists in the routing table and the Switch/Route Processor is NOT needed. This is the default switching option on most Cisco routers though this feature is disabled for diagnostics. If a packet with a new route arrives, then the “process” (hence the term) Process Switching begins again. A check is made against other switching caches.

If an entry exists, then Fast Switching is used.

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Tags: cisco, fast switching, switch processor, cyclic redundancy check, crc