Which Two Protocols Are Link-State Routing Protocols? (Choose Two.)

Introduction

Routing protocols are essential for efficient data transmission in computer networks. Among the different types of routing protocols, link-state routing protocols are known for their ability to maintain a complete and accurate map of the network topology. This article explores which two protocols are link-state routing protocols, their functionalities, advantages, and how they compare to other routing protocols.

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What Are Link-State Routing Protocols?

Link-state routing protocols are a type of interior gateway protocol (IGP) that enables routers to share information about the state of their directly connected links. Unlike distance-vector protocols, which only know about their neighbors, link-state protocols maintain a complete topology database of the network.

Key Characteristics of Link-State Protocols:

1. Topology Awareness: Each router has a full map of the network.

2. Fast Convergence: Updates are sent immediately upon link changes.

3. Shortest Path Calculation: Uses algorithms like Dijkstra’s to determine the best route.

4. Hierarchical Design: Supports large networks efficiently.

Which Two Protocols Are Link-State Routing Protocols?

The two primary link-state routing protocols are:

1. Open Shortest Path First (OSPF)

2. Intermediate System to Intermediate System (IS-IS)

Both protocols are widely used in enterprise and service provider networks. Below, we explore each in detail.

1. Open Shortest Path First (OSPF)

Overview

OSPF is one of the most popular link-state routing protocols, designed for IPv4 and IPv6 networks. It is an open standard, meaning it can be implemented by various vendors.

Key Features of OSPF

• Area-Based Hierarchy: Divides networks into areas to reduce overhead.

• Fast Convergence: Uses Link-State Advertisements (LSAs) to update routers quickly.

• Cost Metric: Determines the best path based on bandwidth.

• Authentication Support: Provides security via MD5 or SHA authentication.

OSPF Operation

1. Neighbor Discovery: Routers form adjacencies using Hello packets.

2. Link-State Database (LSDB) Synchronization: Exchanges LSAs to build a topology map.

3. Shortest Path First (SPF) Calculation: Uses Dijkstra’s algorithm to compute routes.

Advantages of OSPF

• Scalable for large networks
• Supports VLSM and CIDR
• Low network overhead due to incremental updates

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2. Intermediate System to Intermediate System (IS-IS)

Overview

IS-IS is another link-state routing protocol, originally designed for OSI (Open Systems Interconnection) networks but later adapted for IP routing. It is commonly used in ISP and large enterprise networks.

Key Features of IS-IS

• CLNS/IP Support: Works with Connectionless Network Service (CLNS) and IP.

• Two-Level Hierarchy: Uses Level 1 (intra-area) and Level 2 (inter-area) routing.

• Flexible Addressing: Uses NSAP (Network Service Access Point) addresses.

• Efficient Flooding: Uses Sequence Number PDUs (SNPs) for reliable updates.

IS-IS Operation

1. Adjacency Formation: Routers establish neighbors using Hello PDUs.

2. Link-State PDU (LSP) Flooding: Routers exchange LSPs to build the topology.

3. SPF Calculation: Computes the shortest path using Dijkstra’s algorithm.

Advantages of IS-IS

• Highly scalable for large networks
• No IP dependency (works with CLNS)
• Preferred in ISP backbones due to efficiency

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Comparison: OSPF vs. IS-IS

Both protocols are excellent, but OSPF is more common in enterprise networks, while IS-IS dominates ISP environments.

Link-State vs. Distance-Vector Protocols

Link-state protocols (OSPF, IS-IS) differ from distance-vector protocols (RIP, EIGRP) in several ways:

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Why Learn Link-State Protocols for CCNA 200-301?

The CCNA 200-301 exam tests your understanding of:

• OSPF configuration and troubleshooting
• Link-state concepts and advantages
• Differences between routing protocols

By mastering OSPF and IS-IS, you enhance your networking skills and improve career prospects.

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Conclusion

The two link-state routing protocols are OSPF and IS-IS. Both provide fast convergence, scalability, and efficient routing, making them ideal for modern networks.

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Actual Exam Questions For Cisco's 200-301 Study Guide

Sample Questions For Cisco 200-301 Practice Test

1. Which two protocols are link-state routing protocols? (Choose two.)

A) RIP

B) OSPF

C) EIGRP

D) IS-IS

2. Which of the following are link-state routing protocols? (Select two.)

A) BGP

B) OSPF

C) IS-IS

D) RIPv2

3. Identify two link-state routing protocols from the options below.

A) OSPF

B) EIGRP

C) IS-IS

D) RIP

4. Which two routing protocols use the link-state algorithm?

A) OSPF

B) BGP

C) IS-IS

D) IGRP

5. Which pair of protocols are classified as link-state routing protocols?

A) RIP and EIGRP

B) OSPF and IS-IS

C) BGP and RIPv2

D) EIGRP and OSPF