the Forwarding Information Base (FIB) table. The FIB table contains
precomputed reverse lookups and next-hop information for routes,
including the interface and Layer 2 information to use. When a network
topology or routing table change occurs, the change is also reflected in
the FIB table. Figure 17-15 shows CEF switching, where both Layer 2
and Layer 3 information is used.Figure 17-15 CEF Switching: Data Switching Using
FIB Tables
This analogy is sometimes used to illustrate the three
types of switching:
Process switching is like doing math on paper: Write down each step
and solve the problem.
Fast switching, using the route cache, is like solving a problem by hand
once and then simply recalling the answer from memory if the same
problem is given again.
CEF switching is like using formulas in an Excel spreadsheet, and when
the numbers hit the cells, the answer is automatically calculated.Functions of a Router
As discussed earlier in this chapter, a router is a device
that appropriately and efficiently directs traffic from an
incoming interface to the correct outgoing interface.
Apart from this primary function, a router has several
other functions:
Network segmentation: A router uses Network Address Translation
(NAT) to map private IP addresses to public IP addresses. Using NAT
at the router secures the private network and also reduces the number
of IP addresses needed to get messages to the public internet. Figure
17-16 shows how network segmentation is achieved by using NAT.