You should know about the different switching types a router can handle for
the CIT test and for the real world and possibly other exams too. Below is
a copy and paste from Chapter 12 of Top-Down Network Design. The chapter
covers optimizing your network design.
One of the major jobs of a router is to switch packets from incoming
interfaces to outgoing interfaces. Switching involves receiving a packet,
determining how to forward the packet based on the routing topology and QoS
and policy requirements, and switching the packet to the right outgoing
interface or interfaces. The speed at which a router can perform this task
is a major factor in determining network performance in a routed network.
Cisco supports many switching methods, with varying speeds and behaviors.
In general, you should use the fastest switching method available for an
interface type and protocol, (though there are some exceptions to this
guideline.) Using a speedy switching mode is especially important on
backbone and core enterprise routers. Depending on the version of IOS
software you are running, the fastest mode might need to be configured. (It
is not always the default.)
Below are some of the details on the switching types.
Process switching is the slowest of the switching methods. With process
switching, when a packet arrives at an interface, the system processor is
interrupted for the time it takes to copy the packet from the interface
buffer to system memory. The processor looks up the Layer-3 destination
address for the packet in the routing table to determine the exit
interface. The packet is rewritten with the correct header for that
interface and copied to the interface. At this time, an entry is also
placed in the fast-switching cache so that subsequent packets for the
destination address can use the same header. The first packet to a
destination is always process switched.
Fast switching allows higher throughput by switching a packet using an
entry in the fast-switching cache that was created when a previous packet
to the same destination was processed. With fast switching, a packet is
handled immediately, without scheduling an interrupt of the system processor.
Autonomous switching is available on Cisco 7000-series routers and uses an
autonomous-switching cache located on interface processors. Autonomous
switching provides faster packet switching by allowing the ciscoBus
controller to switch packets independently, without having to interrupt the
system processor.
Silicon switching is similar to autonomous switching, but speeds up
autonomous switching through the use of a silicon-switching cache located
on the Silicon Switch Processor (SSP) on some Cisco 7000-series routers.
Optimum switching is similar to fast switching, but is faster, due to an
enhanced caching algorithm, and the optimized structure of the
optimum-switching cache. Optimum switching is only available on routers
equipped with a Route/Switch Processor (RSP).
Distributed switching is supported on routers that include Versatile
Interface Processor (VIP) cards or other interface cards that can receive
route information from the master RSP to make their own autonomous,
multilayer switching decisions. Distributed switching supports very fast
throughput because the switching process occurs on the interface card.
NetFlow switching is a relatively new switching mode that is optimized for
environments where services must be applied to packets to implement
security, QoS features, and traffic accounting. An example of such an
environment is the boundary between an enterprise network and the Internet.
NetFlow switching identifies traffic flows between hosts, and then quickly
switches packets in these flows at the same time that it applies services.
NetFlow switching also lets a network manager collect data on network usage
to enable capacity planning and bill users based on network and application
resource utilization. The data can be collected without slowing down the
switching process.
To maximize network scalability, a good design practice is to use NetFlow
switching on the periphery of a network to enable features such as traffic
accounting, QoS functionality, and security, and to use an even faster
switching mode in the core of the network. At the core of the network, the
switching mode should forward packets based on easily-accessible
information in the packet, and generally should not spend time applying
services.
Hope this helps.
Priscilla
At 06:44 PM 3/4/01, suaveguru wrote:
>hi all,
>
>while studying for the support exam I came across
>Silicon switching, netflow switching , silicon
>switching , Automonous switching , process switching
>etc.
>
>I am confused by so many switching types and what
>routers support them
>
>anyone can give me a quick brief overview and what it
>will be tested on in the exam
>
>thanks
>
>suaveguru
>
>
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