Which of the following incorrectly describes how routing commonly takes place on the Internet?

A.
EGP is used in the areas “between” each AS.

B.
Regions of nodes that share characteristics and behaviors are called ASs.

C.
CAs are specific nodes that are responsible for routing to nodes outside of their region.

D.
Each AS uses IGP to perform routing functionality.

Explanation:
C: A CA, or Certificate Authority, is a trusted third party that provides digital certificates for use in a public key infrastructure. Certificate Authorities have nothing to do with
routing. A PKI environment provides a hierarchical trust model but does not deal with routing of traffic.
A is incorrect because the statement is true. The Exterior Gateway Protocol (EGP) functions between each autonomous system (AS). The architecture of the Internet that
supports these various ASs is created so that no entity that needs to connect to a specific AS has to know or understand the interior protocols that can be used. Instead, for ASs
to communicate, they just have to be using the same exterior routing protocols.
B is incorrect because the statement is true; regions of nodes (networks) that share characteristics and behaviors are called autonomous systems (ASs). These ASs are
independently controlled by different corporations and organizations. An AS is made up of computers and devices, which are administered by a single entity and use a common
Interior Gateway Protocol (IGP). The boundaries of these ASs are delineated by border routers. These routers connect to the border routers of other ASs and run interior and
exterior routing protocols. Internal routers connect to other routers within the same AS and run interior routing protocols. So, in reality, the Internet is just a network made up of
ASs and routing protocols.
D is incorrect because Interior Gateway Protocol (IGP) handles routing tasks within each AS. There are two categories of IGPs: distance-vector routing protocols and linkstate
routing protocols. Distance-vector routing protocols include Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP). Routers using these
protocols do not possess information about the entire network topology. Nodes using link-state routing protocols, on the other hand, possess information about the complete
network topology. Examples of these protocols include Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS).