The Evolution of Cisco Switching

Cisco Systems was founded in 1984 by Len Bosack and Sandy Lerner, a husband and wife team, along with other associates, while working on the network staff of Stanford University. The early Cisco team saw a tremendous commercial application for a multiprotocol router, and literally assembled the devices in their home. While the router was the flagship product, Cisco eventually ventured into other network technology areas, such as Local Area Network (LAN) switches, which it developed through acquisition of existing companies. Here we will examine both the historical and modern methods employed by Cisco switches.

Multilayer switching was developed for the Catalyst 5000/5500 series and extended to early versions of the still-popular 6500 series of chassis-based switching platforms.

Unlike switches that forward packets in hardware, routers perform packet forwarding in software and utilize a completely different set of processes to accomplish the task. While some of the router-based switching methods have used various methods over the years, several types have remained constant, and include process switching and fast switching.

Process Switching: A simple network wants to transmit to a workstation, but using a router as the intervening device rather than a switch.
Fast Switching: Process switching clearly needed a technological “upgrade” in order to reduce demands on the CPU as well as increase router performance. The answer, fast switching, sounds remarkably similar to multilayer switching, but obviously is deployed on a router rather than a switch.

Cisco Express Forwarding, as the name suggests, is a Cisco-proprietary Layer 3 switching technology created for the purpose of enhancing the forwarding and processing of packets through a Cisco device, in this case, switches.

Routing Functionality: While switches originally operated only at the Data-Link (L2) Layer, routing functionality was introduced through the addition of route-processors on the various platforms. Sometimes referred to as the Control Plane, this entity is responsible for the various routing processes, including route table lookup, routing protocol processing, Address Resolution Protocol (ARP) processes, and so forth. Another term used for the routing engine on switches is route processor, or RP.

Data Forwarding Functionality: Enabling the process of CEF switching is identified as the Data Plane and responsible for the forwarding of packets through the switch. Paired closely with the route processor, this logical construct relies on two tables by which it performs its tasks, namely the Forwarding Information Base (FIB) and the Adjacency Table (ADJ).

Forwarding Process: In the CEF equation, first the switch receives a frame with the destination MAC address of the route processor and a CEF table lookup is performed. If no entry exists, a routing table lookup is performed and written to the FIB table for later use. At this point, the frame and packet are rewritten with both the Layer 2 and Layer 3 destination information derived from the FIB and ADJ tables, and then forwarded out the appropriate interface. During the rewrite process, checksums are recalculated, and the IPv4 TTL is decremented by 1. Using this method for processing traffic is far more efficient than earlier methods and gives the equipment the ability to act with much greater speed, freeing CPU resources in the process. Ultimately, CEF switching shortens the entire process of route lookups and packet processing, resulting in more rapid transmission of data.

Knowing the mechanics of switch processing intimately can mean the difference between frustration and success in real-world settings, the very purpose of professional writings such as this one.

Reproduced from Global Knowledge White Paper: The Evolution of Cisco Switches

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