Meta: If you are looking for a network switch for your home or office network with fast
internet connectivity, simply prefer to use the latest Cisco Catalyst switches.
The Types of Switching Methods for Cisco Catalyst Switches
The term Cisco Switching Methods describes the behaviour of the routing processor on
Cisco IOS routers. Since multilayer Cisco Catalyst switches can route and actually contain
routing processes, these concepts need to be reviewed. Cisco IOS-based routers use one of
three packet forwarding methods: process switching, fast switching, and Cisco Express
Forwarding (CEF). In your router study, keep in mind that the switching process is the
slowest form of routing because the router processor has to route and rewrite it using
software. Since the speed and number of cores limit the CPU for routing, this method is not
scalable. The second method, Fast Switching, is a faster method in which the first packet in
the stream is routed and rewritten by the route processor using software and each
subsequent packet is processed by hardware. With a few exceptions, the CEF method uses
hardware redirection tables for the most common traffic flows. When you use CEF, routers
mainly spend their cycles doing other tasks.
The Cisco Catalyst and Nexus switch architectures mainly focus on the Cisco CEF router
equivalent. The switching method as a last resort for Cisco Catalyst or Nexus switches is
process switching. The routing processor on these switches was never designed to redirect
or route packets and therefore this has a detrimental effect on performance. Fortunately,
the default behaviour of this switch is to use Fast Switching, or CEF, and the switching
process is only done when needed.
In Cisco Catalyst switching terminology, fast switching is called route caching, and CEF
distributed hardware forwarding applications are called topology-based switching. As an
overview, the following list summarizes caching and routing based on Cisco Catalyst switch
topology:
Route Caching
Routing caching, also known as flow-based or demand-based switching, describes the layer
3 route cache built into hardware functionality when a switch detects a flow of traffic on the
switch. Functionally, this method is compatible with fast switching in Cisco IOS software.
Topology-based Switching
Information from the routing table is used to populate the route cache regardless of traffic
flow. The cache filled route is FIB and CEF is the facility that creates FIB. This method is
functionally equivalent to CEF in Cisco IOS software.
The following subsections describe route caching and topology-based switching in more
detail.
Route caching is equivalent to fast switching in Cisco Catalyst switches. For route cache to
work, the destination MAC address of the input frame must be from a level 3 switching
interface. The first packet in the stream is routed to the routing processor software because
there is no entry in the cache for the new stream. The routing decisions made by the router
are then programmed into the cache table (hardware routing table) and all subsequent
packets in the stream are routed to the hardware, commonly referred to as an applicationspecific interface scheme (ASIC). Records are only created in the hardware routing table
when the switch detects a new traffic flow and the record expires after not being used for a
certain period of time.
Since entries are only made in the hardware cache as the stream is recognized by the
switch, the route cache always forwards at least one packet to the stream using software.
Route caching has many other names, such as: LAN Net flow redirects, stream or tracebased redirects, and one-time routing, frequent switching.
More About Topology-based Switching
Topology-based switching is a CEF function equivalent to a Cisco Catalyst switch. Topologybased switching is ideal for Layer 3 switching via route caching as it provides the best
performance and scalability. Fortunately, all Cisco Catalyst switches that support Layer 3
routing use topology/CEF-based switching. For CCNP switching, focus primarily on the
benefits and performance of topology-based switching.
CEF uses the information in the routing table to populate the route cache (known as FIB)
without requiring any traffic flow to initiate the caching process. Because these hardware
FIBs exist independently of traffic flows, assuming that the destination address has a route
in the routing table, all packets that are part of the stream are forwarded by the hardware.
FIB even processes the first packet in the stream.
In addition, CEF offers increased support for parallel paths, optimizing IP layer load
balancing. In most current generation Catalyst switches, such as the Catalyst 4500 and 6800,
CEF supports load balancing based on the source IP address and a combination of
destination IP addresses and source and destination IP plus TCP/UDP Port numbers.