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
Explanation:
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.