characteristics, advantages, and drawbacks. It is the role
of the network administrator to decide which protocol to
choose to run on the network and to determine whether
a dynamic routing protocol is even necessary in the first
place. Medium and large networks run at least one of
these protocols on all the routers in the network. The
routers running dynamic routing protocols exchange
data between them and dynamically build their routing
tables based on the information they receive from their
neighbor routers. If multiple routing protocols are
configured on the routers, there needs to be a way to
decide which routes to choose and to populate the
routing table with them. Each routing protocol has an
administrative distance associated with it that is based
on the trustworthiness of the protocol. For example, if a
route for the same network is learned through RIP and
OSPF, the OSPF route will take precedence as OSPF has
a lower administrative distance than RIP. A lower
administrative distance means a higher degree of trust in
that source—so a higher chance of getting added to the
routing table.
Connected routes, meaning the router has an interface
configured in a specific network, have the lowest
administrative distance (AD), 0. Static routes are
preferred next as they usually have an AD of 1. Routing
protocols have different administrative distance values;
for example, BGP has an AD of 20, and RIP has an AD of
- These values for AD are default values, and they can
be changed if additional tweaking of the routing table is
necessary. All dynamic routing protocols mentioned
support routing for both IPv4 and IPv6.
As we’ve seen previously in this chapter, the Internet has
become classless, and there are a large number of
networks and subnets that can be created from those
networks. The same way that a large network address
space can be subdivided into small subnets, the reverse
process is also available; it is called supernetting. This