642-813 Implementing Cisco IP Switched Networks (SWITCH) Practice Test Version 1
QUESTION NO: 1
Company uses layer 3 switches in the Core of their network. Which method of Layer 3 switching
uses a forwarding information base (FIB)?
A. Topology-based switching
B. Demand-based switching
C. Route caching
D. Flow-based switching
E. None of the other alternatives apply
The Layer 3 engine (essentially a router) maintains routing information, whether from static routes
or dynamic routing protocols. Basically, the routing table is reformatted into an ordered list with the
most specific route first, for each IP destination subnet in the table. The new format is called a
Forwarding Information Base (FIB) and contains routing or forwarding information that the network
prefix can reference.
In other words, a route to 10.1.0.0/16 might be contained in the FIB, along with routes to
10.1.1.0/24 and 10.1.1.128/25, if those exist. Notice that these examples are increasingly more
specific subnets. In the FIB, these would be ordered with the most specific, or longest match, first,
followed by less specific subnets. When the switch receives a packet, it can easily examine the
destination address and find the longest match entry in the FIB. The FIB also contains the next-
hop address for each entry. When a longest match entry is found in the FIB, the Layer 3 next-hop
address is found, too.