10 Routing Interview Questions and Answers

Routing is a fundamental aspect of network design and management, playing a crucial role in directing data packets between devices across different networks. Understanding routing protocols, algorithms, and configurations is essential for ensuring efficient and reliable communication within and between networks. Mastery of routing concepts is vital for network engineers, system administrators, and IT professionals who are responsible for maintaining robust and secure network infrastructures.

This article offers a curated selection of interview questions focused on routing, designed to help you demonstrate your expertise and problem-solving abilities. By reviewing these questions and their detailed answers, you will be better prepared to tackle the technical challenges posed during interviews and showcase your proficiency in routing concepts and practices.

Routing Interview Questions and Answers

1. Describe how a router determines the best path for data packets.

Routers determine the best path for data packets using routing tables and algorithms. The routing table contains network topology information, including possible paths to destinations. Each entry includes the destination network, the next hop, and the path’s metric. Routing algorithms, such as Distance Vector and Link State, update and maintain the routing table by considering metrics like hop count, bandwidth, delay, and cost. The router continuously updates its routing table based on information from neighboring routers and the network’s current state.

2. Explain the differences between static and dynamic routing.

Static routing involves manually configuring the routing table with fixed paths to specific networks. It is straightforward and does not change unless manually updated by a network administrator. Static routing is typically used in smaller networks or in scenarios where the network topology is stable.

Dynamic routing uses algorithms and protocols to automatically update the routing table based on network conditions. Protocols like OSPF, BGP, and EIGRP enable routers to share information about network topology changes, ensuring optimal path selection and redundancy.

Key differences include:

• Configuration: Static routing requires manual configuration, while dynamic routing updates automatically.
• Scalability: Static routing is less scalable, suitable for smaller networks, whereas dynamic routing is ideal for larger, complex networks.
• Adaptability: Static routing does not adapt to network changes, while dynamic routing adjusts in real-time.
• Resource Usage: Static routing uses fewer resources, whereas dynamic routing consumes more due to protocol overhead.
• Complexity: Static routing is simpler but cumbersome in large networks, while dynamic routing offers greater flexibility and efficiency.

3. How does OSPF (Open Shortest Path First) protocol work?

OSPF is a link-state routing protocol used within a single autonomous system. It maintains a map of the network topology to determine the shortest path to each destination.

Key features include:

• Link-State Advertisements (LSAs): OSPF routers exchange LSAs to share information about their local state, including connected interfaces and their costs.
• Link-State Database (LSDB): Each OSPF router maintains an LSDB, representing the complete network topology.
• Dijkstra Algorithm: OSPF uses this algorithm to compute the shortest path tree (SPT) from the LSDB.
• Areas and Hierarchies: OSPF supports hierarchical routing by dividing the network into areas, improving scalability and performance.
• Authentication: OSPF supports various methods to ensure the integrity and authenticity of routing information.

4. What are the key differences between BGP (Border Gateway Protocol) and IGP (Interior Gateway Protocol)?

BGP and IGP are routing protocols used in different environments. BGP is an exterior gateway protocol for exchanging routing information between autonomous systems on the internet, known for its scalability and policy-based routing decisions. IGPs, like OSPF and EIGRP, are used within a single autonomous system for fast convergence and efficient routing.

Key differences include:

• Scope: BGP is for inter-domain routing, while IGPs are for intra-domain routing.
• Complexity: BGP is more complex and requires more management effort.
• Transport Protocol: BGP uses TCP, whereas IGPs typically use IP directly.
• Routing Decisions: BGP makes policy-based decisions, while IGPs focus on shortest path.
• Scalability: BGP is highly scalable, whereas IGPs are optimized for smaller routing tables.

5. Explain the concept of route redistribution and when it might be used.

Route redistribution shares routes between different routing protocols, useful when merging networks with different protocols or transitioning from one protocol to another. It involves specifying which routes from one protocol should be redistributed into another, controlled using route maps, access control lists (ACLs), and other filtering mechanisms.

6. Describe the role of MPLS (Multiprotocol Label Switching) in modern networks.

MPLS enhances data packet forwarding by using labels instead of network addresses, creating efficient paths known as Label Switched Paths (LSPs).

Key benefits include:

• Improved Performance: MPLS forwards packets more quickly than traditional IP routing.
• Scalability: It can handle a large number of data flows.
• Quality of Service (QoS): Supports traffic engineering for prioritizing traffic.
• Flexibility: MPLS is transport-agnostic, working over various network infrastructures.
• VPN Support: Used to create Virtual Private Networks (VPNs) for secure connectivity.

7. Explain the differences between IPv4 and IPv6 routing.

IPv4 and IPv6 differ primarily in address space, header complexity, and security features. IPv4 uses a 32-bit address space, while IPv6 uses a 128-bit space, eliminating address exhaustion. IPv6 simplifies headers and includes IPsec as a mandatory feature. It also supports stateless address auto-configuration (SLAAC) and improved multicast and anycast addressing.

8. How does EIGRP (Enhanced Interior Gateway Routing Protocol) work?

EIGRP uses a combination of distance-vector and link-state features, employing the Diffusing Update Algorithm (DUAL) for loop-free and backup routes. It maintains neighbor, topology, and routing tables, using a composite metric based on bandwidth, delay, load, and reliability. EIGRP supports unequal-cost load balancing and uses Hello packets to maintain neighbor relationships.

9. Describe the process of route summarization and its benefits.

Route summarization combines contiguous IP addresses into a single route, reducing the number of routes advertised to other networks.

Benefits include:

• Reduced Routing Table Size: Fewer entries conserve memory and processing power.
• Improved Network Performance: Smaller tables lead to faster route lookups and reduced CPU usage.
• Decreased Bandwidth Usage: Fewer routes reduce routing information exchange.
• Enhanced Stability: Summarized routes help isolate network issues, improving stability.

10. What is the role of administrative distance in routing decisions?

Administrative distance (AD) indicates the trustworthiness of routes learned via different protocols. When a router learns about the same destination from multiple sources, it uses the AD to select the best path. Lower AD values indicate more preferred routes, helping maintain a consistent and efficient routing table.