Routing protocol selection — classless support with distance-vector behavior A network administrator must configure a router with a distance-vector (or advanced distance-vector) routing protocol that fully supports classless routing with VLSM and CIDR summarization. Which single protocol meets these requirements?

Introduction / Context:
Choosing the correct interior gateway protocol (IGP) depends on capabilities such as classless routing, route summarization, and operational model (distance-vector vs. link-state). Many legacy protocols are classful and cannot carry variable-length subnet masks (VLSM). This question tests recognition of which distance-vector family protocol supports classless routing in modern IPv4 designs.

Given Data / Assumptions:

• We need a distance-vector or advanced distance-vector protocol.
• The protocol must support classless routing, VLSM, and CIDR.
• Candidate protocols listed include IGRP, OSPF, RIPv1, EIGRP, and IS-IS.

Concept / Approach:
Classless routing protocols carry subnet mask information in updates and therefore support VLSM and CIDR. RIPv1 and IGRP are classful (no mask in updates). EIGRP is an advanced distance-vector (sometimes called hybrid) that includes mask information and supports VLSM and CIDR. OSPF and IS-IS are link-state, not distance-vector. Therefore, among distance-vector options, EIGRP is the correct choice.

Step-by-Step Solution:
Identify classful protocols: RIPv1 and IGRP → no VLSM/CIDR support.Identify classless protocols: EIGRP, OSPF, IS-IS → include mask in advertisements.Filter by distance-vector family: EIGRP qualifies; OSPF/IS-IS are link-state.Conclude that EIGRP uniquely satisfies both conditions.

Verification / Alternative check:
Configuration guides show EIGRP supports route summarization, VLSM, unequal-cost load balancing, and includes the subnet mask in its updates. In contrast, IGRP and RIPv1 lack mask carriage and are deprecated for classless designs.

Why Other Options Are Wrong:

• IGRP: Classful; cannot carry masks, no VLSM/CIDR.
• OSPF: Classless but link-state, not distance-vector.
• RIPv1: Classful; no subnet mask in updates.
• IS-IS: Classless link-state; not distance-vector.

Common Pitfalls:
Confusing protocol families (calling EIGRP link-state) or assuming all modern protocols are link-state. Also, mixing up RIPv2 (classless) with RIPv1 (classful).

Final Answer:
EIGRP