CISCO CCNA 200-301 Q150

Which of the following are classless routing protocols? (Choose four.)

A. Open Shortest Path First (OSPF)
B. Enhanced Interior Gateway Routing Protocol (EIGRP)
C. Interior Gateway Routing Protocol (IGRP)
D. Routing Information Protocol version 1 (RIPv1)
E. Border Gateway Protocol (BGP)
F. Routing Information Protocol version 2 (RIPv2)

Correct Answer: A, B, E, F

Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol (BGP), and Routing Information Protocol version 2 (RIPv2) are classless routing protocols.
Intermediate-System-to-Intermediate System (IS-IS) is also a classless routing protocol. The options IGRP and RIPv1 are incorrect because these are classful routing protocols.

The following are characteristics of classless routing protocols:
– The subnet mask is advertised with each route by using classless routing protocols.
– Flexible route summarization and supernetting (CIDR) are allowed in classless routing protocols.
– Classless routing protocols support variable length subnet masks (VLSM), which allow different subnets of a given IP network to be configured with different subnet masks.

One of the main advantages of using a classless routing protocol is its ability to minimize the effects of discontiguous networks. When subnets of the same classful network are separated by another classful network, the networks are called discontiguous.

Examine the diagram below:

The LAN networks extending from Router A and Router B are derived from the same Class C network, A classful routing protocol such as RIP v1 would not be able to determine the direction to send the packets, but since classless protocols include the subnet mask in advertisements, they would not suffer the same problem. Whenever networks with non-default subnet masks are used, a classless routing protocol will be required.
Below are some examples of networks that do not have default masks. You can recognize them by the fact that they are not /8, /16, or /24.

All of the classless protocols discussed here are interior routing protocols with the exception of Border Gateway Protocol (BGP), which is an external routing protocol used to connect different autonomous systems. For example, BGP would be used to connect two OSPF autonomous systems (AS).