How To Choose The Best Router Switching Path For Your Network
Introduction
There are a plethora of switching paths available to various Cisco routers and Cisco IOS releases. Which is the best one for your network, and how do they all work? This white paper is an attempt to explain each of the following switching paths so you can make the best decision about which switching path fits your network.
First, examine the forwarding process itself. There are three steps to forwarding a packet through a router:
- Determine if the packet's destination is reachable.
- Determine the next hop toward the destination, and the interface through which that next hop is reachable.
- Rewrite the Media Access Control (MAC) header on the packet so it will successfully reach its next hop.
Each of these steps is critical for the packet to reach its destination.
Note: Throughout this document, we use the IP switching path as an example; virtually all the information provided here is applicable to equivalent switching paths for other protocols, if they exist.
Process Switching
Process switching is the lowest common denominator in switching paths; it is available on every version of IOS, on every platform, and for every type of traffic being switched. Process switching is defined by two essential concepts:
- The forwarding decision and information used to rewrite the MAC header on the packet are taken from the routing table (from the routing information base, or RIB) and the Address Resolution Protocol (ARP) cache, or from some other table that contains the MAC header information mapped to the IP address of each host that is directly connected to the router.
- The packet is switched by a normal process running within IOS. In other words, the forwarding decision is made by a process scheduled through the IOS scheduler and running as a peer to other processes on the router, such as routing protocols. Processes that normally run on the router aren't interrupted to process switch a packet.