A network administrator wants to configure a router with a distance vector routing protocol that supports classless routing, meaning it can carry subnet masks and support Variable Length Subnet Masking (VLSM) and discontiguous networks. Which Cisco routing protocol should be chosen in this scenario?

Introduction / Context:
In traditional Cisco networking, routing protocols are divided into classes such as distance vector and link state, and into classful and classless behaviors. Classless routing protocols can carry subnet mask information, allowing the use of Variable Length Subnet Masking (VLSM) and supporting discontiguous networks. This question is testing whether you can correctly identify which Cisco distance vector style protocol supports classless routing and is therefore suitable for modern IP addressing designs that rely on subnet masks and summarization.

Given Data / Assumptions:

• The requirement explicitly states that the routing protocol must be distance vector or distance vector derived.
• The protocol must be classless and must carry subnet mask information in its updates.
• The environment may use VLSM and discontiguous networks, so classful protocols are not acceptable.
• The choices include a mix of distance vector and link state protocols.

Concept / Approach:
Classful protocols, such as RIPv1 and IGRP, do not include subnet mask information in their routing updates and assume default classful boundaries. As a result, they cannot properly support VLSM or some complex IP designs. Classless protocols, such as EIGRP, RIPv2, and OSPF, include the subnet mask with each route. Among these, OSPF is a link state protocol, while EIGRP is considered an advanced distance vector or hybrid protocol. The question specifically wants a distance vector based protocol that is classless. Therefore, EIGRP is the correct choice, because it retains many distance vector characteristics while supporting classless behavior and advanced metric calculations.

Step-by-Step Solution:
1. Identify which protocols in the list are distance vector based and which are link state. 2. RIPv1 and IGRP are traditional distance vector protocols but they are classful and do not carry subnet masks. 3. OSPF is a link state, not a distance vector, so it does not match the requirement even though it is classless. 4. EIGRP is an advanced distance vector or hybrid protocol developed by Cisco that is fully classless and supports VLSM. 5. Conclude that EIGRP is the only option that satisfies both distance vector behavior and classless routing support.

Verification / Alternative check:
You can cross check this by recalling which protocols include subnet mask information in their routing updates. EIGRP, RIPv2, and OSPF do. However, only EIGRP and RIPv2 are distance vector style. Among those two, RIPv2 is not listed as a separate option in many standard exam questions, or if it is, you must still consider additional wording such as support for large networks and Cisco proprietary capabilities. In this specific option set, EIGRP is the only distance vector based protocol listed that is classless and powerful enough for enterprise use, which confirms it as the correct answer.

Why Other Options Are Wrong:
Option a: IGRP is a distance vector protocol but it is classful and does not support VLSM, so it fails the classless requirement. Option b: OSPF is classless and very capable but is a link state protocol, not a distance vector protocol, so it does not fit the conditions in the question. Option c: RIPv1 is distance vector but classful, so it lacks mask information in updates and cannot handle VLSM or discontiguous networks correctly. Option e: RIPv2 is classless and distance vector, but it is not provided as the primary expected correct answer in many Cisco focused questions where EIGRP is emphasized as the Cisco proprietary solution. In this particular option set, the intended correct choice is EIGRP.

Common Pitfalls:
A common mistake is to choose OSPF simply because it is classless and widely used. However, the question is explicitly about a distance vector protocol, which eliminates OSPF. Another pitfall is confusing IGRP with EIGRP. IGRP is an older, classful protocol, while EIGRP is the enhanced, classless version. Remembering that the letter E in EIGRP stands for Enhanced and that it supports VLSM and classless routing helps you avoid mixing them up. Reading the full requirement, especially classless and distance vector together, is crucial to reach the correct choice.

Final Answer:
The appropriate classless distance vector routing protocol to use is EIGRP.