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Chapter 4

Cisco's Diagnostic Commands

Fast switching is possible on these routers due to the shared memory on

the processor board. The specifics of the different switching paths will be
discussed next; for now, we'll just say that fast switching is used on the
4000/2500 series routers.

Now that we have discussed the switching types supported by various

router platforms, we will explain the different switching methods in detail.

Switching Methods

The

switching path

is the logical path that a packet follows when it's switched

through a router. As we have discussed, there are many types of switching and
it is important not to confuse them. This section explains methods used
by routers to move a packet from an incoming interface to the correct outgoing
interface. By using switching paths, extra lookups in route tables are elimi-
nated, and processing overhead is reduced.

The router's physical design and its interfaces allow for a variety of

switching processes on the router. This frees up the processor to focus on
other tasks, instead of looking up the source and destination information for
every packet that enters the router.

We have already discussed router architecture, so we will focus directly on

the details of each switching type. The most processor-intensive method is
discussed first; we end with the most efficient method of switching.

Process Switching

Process switching

is the action of the processor having to determine the exiting

interface for every packet. As a packet arrives on an interface to be forwarded,
eventually it is copied to the router's process buffer, and the router performs
a lookup on the Layer 3 address (

eventually

means that there are a few

steps before the packet is copied to the route processor buffer). Using the route
table, an exit interface is associated with the destination address. The proces-
sor encapsulates and forwards the packet with the added new information to
the exit interface while the router initializes the fast-switching cache.
Subsequent packets that require process switching and are bound for the same
destination address follow the same path as the first packet.

Overhead ensues because the processor is occupied with Layer 3 look-

ups--determining which interface the packet should exit from and calculat-
ing the CRC for the packets. If every packet required all of that to be routed,
the processor could get really bogged down. The answer is to use other types
of switching whenever and wherever possible.

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