Router Switching Modes


Background.  When designing routed networks it can be very important to consider what sort of switching/forwarding methods are used by a router.  Remember, that most packets are part of a much larger message and many routes are frequently used.

The following listing presents a brief overview.


Process Switching The router looks up the layer 3 address in the packet and then uses the routing table located in main memory to associate this address with a destination network or subnet.

Process switching is actually a scheduled process and gets delayed both by the schedule and the latency within the process.


Fast Switching An incoming packet is matched to an entry in the fast switching or route cache located in main memory.  This cache is populated when the first packet in a message is process switched. 

Fast switching is done via asynchronous interrupts.  It has several advantages.

  • It uses a cache created by previous packets
  • It runs at an interrupt level
  • The route cache is usually much smaller than a routing table, so searches are faster
Autonomous Switching Here an incoming packet is matched to an entry in the autonomous switching cache located on the interface processor.  This provides faster packet switching by allowing the bus controller (likely Cisco) to switch packets independently without having to interrupt the system processor.


Silicon Switching Here an incoming packet is matched to an entry in the silicon switching cache located in the SSE - Silicon Switching Engine of the SSP - Silicon Switching Processor.  This improves on the others by allowing the SSE to switch packets independently without needing to interrupt the system processor.


Optimum Switching This is available on the RSP - Router/Switch Processor in one line of Cisco routers.  It is even faster than fast switching, though it doesn't have a dedicated switching engine such as SSE.  This is due to enhancements in the cache data structure and caching algorithm, the use of certain interface processors to perform packet classification and the RSP module.


Distributed Switching If the router has an RSP along with VIP - Versatile Interface Processor controllers, the VIP hardware can be configured to switch packets received by the VIP with no par-packet intervention by the RSP.


NetFlow Switching This identifies traffic flows between internetwork hosts and on a connection oriented basis.  It switches these packets at the same time it applies relevant services such as security, QoS and traffic accounting.


Process switching and fast switching are what are typically found on small or medium sized networks.

In the past, most routers were selected based on pps - packets per second.  Now it is very important to also consider other factors such as security needs and quality of service.