Border Gateway Protocol is the backbone of Internet routing and a favorite topic in network interviews. Interviewers use BGP questions to test not only protocol knowledge, but also how well you understand large-scale network design, routing policies, and real-world traffic behavior. Concepts like route advertisement, AS path selection, and peering decisions often separate theoretical understanding from practical expertise.
This blog is designed as an interview-focused guide to BGP routing. The questions and answers are written in a clear, simple style, making them easy to revise before interviews. Whether you are preparing for enterprise networking roles, service provider discussions, or cloud networking interviews, this guide will help you understand how BGP actually works in real Internet routing scenarios.
Interview Questions and Answers
1. What is BGP and why is it important for Internet routing?
Answer: BGP is a path-vector routing protocol used to exchange routing information between autonomous systems. It is responsible for deciding how data travels across the Internet by selecting the best available paths between networks.
BGP is critical because it enables independent networks to connect, scale globally, and apply routing policies rather than relying only on shortest paths.
2. What is an autonomous system in BGP?
Answer: An autonomous system is a collection of IP networks managed by a single administrative entity that follows a common routing policy. Each autonomous system is identified by a unique AS number.
BGP uses these AS numbers to track routing paths and prevent routing loops during Internet routing.
3. How does BGP differ from internal routing protocols?
Answer: BGP focuses on policy-based routing between autonomous systems, while internal routing protocols focus on finding the shortest path within a single network.
Instead of metrics like bandwidth or delay, BGP relies on attributes such as AS path, local preference, and route advertisement policies.
4. What is route advertisement in BGP?
Answer: Route advertisement is the process by which a BGP router announces reachable network prefixes to its neighbors. These advertisements include path attributes that help neighbors decide whether to accept and prefer the route.
Effective route advertisement is essential for controlling traffic flow and visibility on the Internet.
5. What is the AS path attribute and why is it important?
Answer: The AS path is a list of autonomous systems that a route has traversed to reach a destination. BGP uses the AS path to prevent routing loops and to influence route selection.
Shorter AS paths are generally preferred, making this attribute a key factor in Internet routing decisions.
6. How does BGP select the best path?
Answer: BGP selects the best path by evaluating attributes in a specific order, starting with local preference, then AS path length, origin type, and other factors. This step-by-step decision process allows network operators to influence routing behavior through policies rather than relying on default metrics.
7. What is BGP peering?
Answer: BGP peering is a relationship between two autonomous systems where they exchange routing information. Peering can be established for traffic optimization, cost reduction, or redundancy. Peering decisions directly affect performance, latency, and resilience in Internet routing.
8. What is the difference between internal BGP and external BGP?
Answer: Internal BGP is used to exchange routing information within the same autonomous system, while external BGP is used between different autonomous systems. External BGP handles Internet-facing routes, whereas internal BGP ensures consistent route distribution inside large enterprise or service provider networks.
9. Why does BGP require full mesh in internal deployments?
Answer: Internal BGP requires full mesh connectivity to ensure that all routers receive consistent routing information without loops. In large networks, route reflectors are commonly used to reduce the complexity of full mesh designs.
10. What are route reflectors in BGP?
Answer: Route reflectors allow BGP routes learned from one internal peer to be shared with others without requiring full mesh connectivity. They simplify scaling internal BGP while maintaining stable Internet routing behavior.
11. How does BGP handle scalability?
Answer: BGP scales by summarizing routes, filtering advertisements, and using hierarchical designs such as route reflectors and confederations. These mechanisms allow BGP to manage extremely large routing tables across global networks.
12. What is route filtering and why is it important?
Answer: Route filtering controls which prefixes are advertised or accepted by a BGP router. It prevents accidental route leaks and limits unnecessary routing information. Proper filtering is essential for stable and secure Internet routing.
13. How does BGP support traffic engineering?
Answer: BGP supports traffic engineering by allowing operators to influence route selection using attributes such as local preference, AS path prepending, and selective route advertisement. This flexibility helps enterprises and providers optimize inbound and outbound traffic flows.
14. What is AS path prepending?
Answer: AS path prepending is a technique where an autonomous system artificially lengthens its AS path to make a route less preferred. It is commonly used to control inbound traffic patterns in Internet routing.
15. What is the role of BGP in enterprise networks?
Answer: In enterprise environments, BGP is used for multi-homing, redundancy, and integration with cloud and service provider networks. It allows enterprises to control routing policies while maintaining high availability.
16. How does BGP behave during network failures?
Answer: When a failure occurs, BGP withdraws affected routes and advertises alternative paths. This convergence process can take time compared to internal routing protocols. Despite slower convergence, BGP prioritizes stability over rapid changes in Internet routing.
17. What is BGP convergence?
Answer: BGP convergence is the process by which routers reach a consistent view of routing paths after a change. Good network design and filtering help reduce convergence time and prevent routing instability.
18. How does BGP prevent routing loops?
Answer: BGP prevents loops by checking the AS path attribute. If a router sees its own AS number in the path, it rejects the route. This simple mechanism is highly effective in large-scale Internet routing.
19. What are common BGP misconfiguration risks?
Answer: Common risks include incorrect route advertisement, missing filters, and accidental acceptance of invalid prefixes. Such issues can lead to traffic hijacking or widespread routing disruptions.
20. Why is BGP considered policy-driven?
Answer: BGP allows operators to define how routes are selected, advertised, and preferred based on business or technical requirements. This policy-driven nature makes BGP flexible but also requires careful planning and expertise.
Conclusion
BGP is more than just a routing protocol; it is the foundation of Internet routing and inter-network connectivity. Interviewers expect candidates to understand how BGP uses route advertisement, AS path evaluation, and peering strategies to move traffic reliably across large networks.
By focusing on both theory and practical behavior, you can confidently explain how BGP supports scalable, resilient, and policy-based routing. Mastering these concepts will strengthen your performance in networking and infrastructure interviews.
