Multilayer switching includes the ability to switch data based on
information at multiple layers. Multilayer switching also refers to a class of
high-performance routers that provide Layer 3 services and simultaneously
forward packets at wire-speed through switching hardware. A Layer 3 switch
performs packet switching, route processing and intelligent network
services.
Layer 3 switch processing forwards packets at wire-speed by
using ASIC hardware instead of microprocessor-based engines as might be found
on a traditional router. Specific Layer 3 components such as routing tables or
ACLs are cached into hardware. The Layer 3 packet headers of data traffic will
be analyzed and packets forwarded at line speeds based upon that cached
information.
Layer 3 switching can occur at two different locations on
the switch:
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Centralized switching – Switching decisions are made on the route
processor by a central forwarding table, typically controlled by an ASIC.
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Distributed switching – Switching decisions can be made on a port or
line card level rather than on a central route processor. Cached tables are
distributed and synchronized to various hardware components so processing can
be distributed throughout the switch chassis.
Layer 3 switching takes place using one of these methods, which are
platform dependent:
-
Route caching – Also known as flow-based or demand-based switching,
a Layer 3 route cache is built in hardware as the switch sees traffic flows
into the switch.
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Topology-based switching – Information from the routing table is
used to populate the route cache regardless of traffic flow. The populated
route cache is called the Forwarding Information Base. Cisco Express Forwarding
is the facility that builds the FIB