Border Gateway Protocol (BGP) is often called the backbone of the internet — it is the routing protocol that decides how packets move efficiently between networks across the globe. BGP is used by Internet Service Providers (ISPs), large enterprises, and cloud networks to exchange routing information between Autonomous Systems (AS) and maintain stable, policy-driven connectivity.
For network engineers, BGP is one of the most critical concepts to master. Whether you are preparing for interviews at telecom companies, data centers, or cloud infrastructure roles, you will almost certainly face questions on BGP — from simple definitions to complex routing scenarios. Interviewers often use BGP questions to assess your understanding of routing policies, traffic engineering, and your ability to troubleshoot real-world network issues. This blog compiles the Top 50 BGP Interview Questions and Answers, covering basic, intermediate, and advanced topics.
Target Audience
This blog is designed for professionals and learners who want to strengthen their understanding of BGP (Border Gateway Protocol) for interviews and practical networking roles. It is particularly useful for:
- Network Engineers and Administrators preparing for technical interviews at ISPs, data centers, or enterprise IT firms.
- NOC (Network Operations Center) Analysts looking to upgrade their knowledge of routing and connectivity management.
- Cloud and Infrastructure Engineers working with hybrid or multi-cloud environments where BGP plays a key role in routing and redundancy.
- Students and Certification Candidates pursuing Cisco, Juniper, or cloud certifications such as CCNA, CCNP, CCIE, or AWS Advanced Networking.
- IT Professionals transitioning into networking roles who want to grasp both the theoretical and practical aspects of BGP.
Whether you are preparing for your first networking interview or refining your expertise for a senior-level role, this guide will help you understand, explain, and troubleshoot BGP concepts with real-world clarity and confidence.
Section 1: Basic Scenario-Based Border Gateway Protocol (BGP) Interview Questions and Answers (For Freshers)
This section focuses on simple but realistic networking situations where BGP plays a key role. Each question is designed to test your practical understanding of how BGP behaves in everyday network setups rather than just theoretical definitions.
1. Question: You are setting up two routers between different ISPs and need them to exchange routing information. Which protocol should you use and why?
Answer: You should use BGP (Border Gateway Protocol) because it is specifically designed to exchange routing information between different autonomous systems (AS). It allows you to control routing policies and ensures that data travels across the internet efficiently and securely.
2. Question: Your organization is connected to two ISPs for redundancy. How does BGP help in this setup?
Answer: BGP enables multi-homing, where your network connects to multiple ISPs. It allows you to advertise your network to both providers and receive routes from them. BGP can then intelligently select the best or preferred path for outgoing and incoming traffic using attributes like LOCAL_PREF or AS_PATH.
3. Question: During router configuration, you are asked to enter an “AS number.” Why is it required in BGP?
Answer: Every BGP router belongs to an Autonomous System (AS), identified by a unique AS number. This number distinguishes your network from others on the internet and allows BGP to track the path a route has taken. It’s essential for loop prevention and policy enforcement between different networks.
4. Question: You configure two routers with BGP, but they are in the same organization. Should you use eBGP or iBGP, and why?
Answer: You should use iBGP (Internal Border Gateway Protocol (BGP)) because both routers belong to the same AS. iBGP is used to share external routing information across routers within the same organization. eBGP, on the other hand, is used when routers belong to different AS numbers.
5. Question: A new Border Gateway Protocol (BGP) session is stuck in the “Active” state. What could be wrong?
Answer: The “Active” state means the router is trying to establish a TCP connection but failing. Common reasons include incorrect neighbour IP, mismatched AS number, TCP port 179 being blocked by a firewall, or the remote router not configured yet. Checking connectivity and configuration usually resolves this.
6. Question: You notice that routes from one neighbour are not appearing in the BGP table. What could be the reason?
Answer: The most common reasons are that the network command is missing, the route is not present in the routing table, or the neighbour has applied filters preventing advertisement. Verifying prefix advertisements and using show ip bgp helps identify the issue.
7. Question: You see two routes to the same destination in your BGP table. How does BGP decide which one to use?
Answer: BGP applies its path selection algorithm, preferring routes with higher LOCAL_PREF, shorter AS_PATH, lower origin code, lower MED, and finally, the lowest router ID. This ensures that routing decisions follow business or performance policies.
8. Question: A colleague tells you that BGP uses hop count to find the best route. Is that true?
Answer: No, unlike protocols such as RIP, BGP does not use hop count. Instead, it uses attributes like AS_PATH and LOCAL_PREF to make policy-based decisions, offering more control over routing behaviour.
9. Question: You are configuring eBGP between two routers that are not directly connected. The session is not coming up. What should you do?
Answer: eBGP expects peers to be directly connected with a TTL of 1. For non-directly connected routers, you need to use the ebgp-multihop command to increase the TTL value and allow the session to form successfully.
10. Question: You configured BGP but are unsure if it is working properly. What command would you use to verify it?
Answer: Use the command show ip bgp summary to verify neighbour status, number of routes exchanged, and session state. If the state shows Established, it means the BGP session is active and functioning correctly.
11. Question: You want to advertise only specific networks from your router into BGP. How do you ensure that?
Answer: Use the network command with the exact prefix and mask that exist in the routing table. BGP will only advertise routes that are present in the local routing table and match the configuration exactly.
12. Question: You configured iBGP between two routers, but routes are not being shared with another internal router. What could be the reason?
Answer: iBGP requires a full mesh by default, meaning each iBGP router must form a session with every other iBGP router. If one router is not directly peered, routes will not propagate unless a route reflector is used.
13. Question: You see a Border Gateway Protocol (BGP) route with your own AS number in its AS_PATH. What does this indicate?
Answer: It means a routing loop has been detected. BGP automatically rejects routes that contain its own AS number in the AS_PATH to prevent loops between networks.
14. Question: A junior engineer suggests using BGP for internal LAN routing. Would you agree?
Answer: No. BGP is designed for inter-domain routing, not small internal LANs. For internal routing, protocols like OSPF or EIGRP are more efficient. BGP is best suited for large-scale networks and connections between different ASes.
15. Question: You are connecting your enterprise network to AWS or Azure using VPN or Direct Connect. Why is BGP preferred in this setup?
Answer: BGP dynamically exchanges routes between your on-premise network and the cloud. It simplifies routing management, supports redundancy, and automatically updates routes when the network changes — unlike static routes that require manual configuration.
These basic scenario-based questions help you connect BGP theory with real-life use cases. Understanding these will allow you to confidently explain how BGP behaves in typical enterprise or ISP environments.
Section 2: Intermediate Scenario-Based BGP Interview Questions and Answers (For Network Engineers)
This section covers medium-difficulty, real-world BGP scenarios that test your practical understanding of configuration, path selection, redundancy, and troubleshooting in enterprise and ISP environments.
1. Question: You have configured Border Gateway Protocol (BGP)between two routers, but the session keeps going down after a few minutes. What could be causing this instability?
Answer: Frequent BGP session drops usually indicate connectivity or timer issues. The most common reasons include mismatched keepalive/hold timers, network congestion, or intermediate devices blocking TCP port 179. Reviewing logs and verifying timers with the show ip bgp neighbors command can help identify the cause.
2. Question: Your company is connected to two ISPs, and you notice that most outbound traffic is going through one ISP even though both links are active. How can you balance the load?
Answer: You can configure LOCAL_PREF values to influence outbound routing. By assigning equal LOCAL_PREF values to both ISPs, traffic can be balanced. For more granular control, use route-maps to assign different preferences based on destination prefixes or traffic type.
3. Question: You observe that some incoming traffic from the internet is favouring one ISP over another. How can you influence inbound routing?
Answer: Inbound routing can be influenced using AS_PATH prepending or MED (Multi-Exit Discriminator). By artificially lengthening the AS_PATH or increasing MED for specific routes, you make one path less preferred to external peers. However, ISPs may ignore MED unless explicitly agreed upon.
4. Question: One of your BGP routers shows routes in the BGP table but not in the routing table. Why is that happening?
Answer: This happens when the route has a better administrative distance from another protocol (like OSPF) or when synchronization is enabled. BGP routes only install in the routing table if no better route exists. Disabling synchronization or adjusting administrative distance can resolve this.
5. Question: A BGP peer is configured correctly, but the session remains in the “Idle” state. What could be wrong?
Answer: The “Idle” state indicates that the router is not even attempting to establish a TCP session. Common causes include wrong neighbour IP address, missing neighbor remote-as command, or an access control list (ACL) blocking TCP port 179.
6. Question: You notice frequent route flapping in your BGP network. What steps would you take to stabilize it?
Answer: Implement route dampening to suppress unstable routes temporarily. Also, investigate the root cause — such as poor link quality, interface flaps, or unstable upstream peers — and address the physical or configuration issues causing instability.
7. Question: A new router has been added to your AS, but it cannot learn routes from existing iBGP peers. What could be the reason?
Answer: By default, iBGP does not advertise routes learned from one iBGP peer to another. You need to implement a route reflector or full mesh configuration so that routes are properly propagated within the AS.
8. Question: Two eBGP routers are connected through multiple links, but only one link is being used. How can you use both links simultaneously?
Answer: You can enable BGP multipath to allow multiple equal-cost routes in the routing table. The command maximum-paths [number] under BGP configuration ensures that both links share the load for outgoing traffic.
9. Question: A customer reports that their network prefixes are not being advertised to upstream providers. What would you check first?
Answer: Verify that the prefixes are present in the routing table and that the network command or redistribution is configured correctly in BGP. Ensure that no outbound route-map or prefix-list is filtering those prefixes.
10. Question: You are configuring iBGP between routers using loopback interfaces, but the session is not coming up. What might be missing?
Answer: When using loopback interfaces, you must configure the update-source loopback command and ensure the remote peer is reachable. For eBGP sessions, you may also need to enable ebgp-multihop to increase the TTL value.
11. Question: Your network uses multiple route reflectors, but some routes are missing from clients. What could cause this?
Answer: Route reflectors must be fully meshed with each other. If two route reflectors are not exchanging routes, their clients will not receive the complete routing information. Configuring reflector-to-reflector peering resolves the issue.
12. Question: One of your peers is advertising private IP addresses over eBGP, which should not happen. How do you prevent this?
Answer: Apply a prefix-list or route-map on outbound advertisements to filter private IP ranges (like 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16). This ensures only valid public prefixes are advertised to external peers.
13. Question: A Border Gateway Protocol (BGP) session between two routers in different data centers is taking too long to establish. How can you troubleshoot this?
Answer: Check for network latency or firewall filtering on TCP port 179, verify that MTU settings are consistent, and ensure ebgp-multihop is configured if the routers are not directly connected. Monitoring BGP debug logs helps pinpoint the issue.
14. Question: You are performing maintenance and need to temporarily stop BGP advertisements without shutting down the session. How can you do this?
Answer: Use the neighbor [IP] shutdown command (Cisco IOS 15.1+) or apply an outbound route-map with a “deny” statement to stop route advertisements temporarily while keeping the session active.
15. Question: Your organization’s BGP routes are leaking into an internal OSPF domain. What would you do?
Answer: Review redistribution policies. You can prevent unintended redistribution by using route filtering, route tagging, or by controlling which BGP routes are injected into IGPs through specific route-maps or distribute-lists.
These intermediate-level scenarios test your ability to think practically, apply routing logic, and troubleshoot network issues efficiently. Mastering these concepts shows that you can maintain stable and policy-driven BGP environments.
Section 3: Advanced Scenario-Based BGP Interview Questions and Answers (For Senior Network Professionals)
This section explores advanced, real-world scenarios that test your ability to design, optimize, and troubleshoot BGP in complex enterprise and ISP networks. These questions are meant for senior engineers who manage routing policies, scalability, and network security at a global level.
1. Question: Your network has hundreds of internal routers running iBGP. Maintaining a full mesh of peerings is becoming unmanageable. How would you solve this?
Answer: Implement route reflectors to simplify iBGP scalability. A route reflector allows certain routers (clients) to share routes through a central node instead of peering with every other router. This drastically reduces configuration complexity while maintaining full route visibility.
2. Question: You have a large AS divided across multiple geographical regions. iBGP scalability is still an issue, even with route reflectors. What can you do?
Answer: Use BGP confederations. Confederations divide a large AS into smaller sub-ASes internally, each operating as an independent BGP system. Externally, these sub-ASes appear as a single AS, reducing session overhead while keeping route policies flexible.
3. Question: A customer reports that their prefixes are intermittently disappearing from the global routing table. What could cause this?
Answer: This could be due to route flapping — frequent withdrawal and re-advertisement of routes. You can stabilize the network by implementing BGP route dampening, which suppresses unstable routes temporarily until they become stable again.
4. Question: Your organization connects to multiple ISPs. During peak hours, one link becomes congested while others are underutilized. How can you optimize traffic distribution?
Answer: Use Border Gateway Protocol (BGP) traffic engineering by manipulating attributes:
- Increase LOCAL_PREF to prefer specific outbound paths.
- Adjust MED or AS_PATH prepending to influence inbound traffic.
Additionally, use BGP multipath to enable load-sharing across equal-cost links.
5. Question: A route reflector in your network fails. How do you ensure that BGP connectivity continues without major impact?
Answer: Deploy redundant route reflectors and configure each client with multiple reflectors. Route reflectors do not form a single point of failure if redundancy is built in. Clients will maintain stable sessions through the backup reflector automatically.
6. Question: Your BGP routers are receiving more routes than expected from a peer, leading to memory overload. How do you handle this?
Answer: Apply maximum-prefix limits on the BGP session. This command caps the number of prefixes that can be received from a neighbour. If the limit is exceeded, the router either shuts down the session or issues a warning, protecting it from overload.
7. Question: During a global routing audit, you discover that your network is advertising private IP addresses to external peers. How do you prevent this?
Answer: Apply outbound prefix filtering using prefix-lists or route-maps to block private address ranges (RFC1918). Additionally, enforce route policy templates across all edge routers to prevent human error or misconfiguration in future.
8. Question: You need to perform maintenance on one BGP router without causing downtime. How can you gracefully redirect traffic?
Answer: Use BGP graceful shutdown by lowering LOCAL_PREF or applying a route-map to make alternate routers more preferred. This allows traffic to reroute naturally before you bring down the session, ensuring zero packet loss during maintenance.
9. Question: Two data centers use BGP for interconnection. After a link failure, traffic reroutes through a backup path, but the failover takes too long. How can you improve convergence time?
Answer: Enable BGP fast failover and tune BFD (Bidirectional Forwarding Detection) for quicker detection of link failures. BFD provides sub-second detection, allowing BGP to withdraw and reroute traffic much faster than default timers.
10. Question: You need to prevent one of your upstream ISPs from using your network as a transit path for other networks. What configuration should you use?
Answer: Apply the no-export BGP community to your outbound advertisements. This ensures your routes are visible only to directly connected peers and not propagated further. Alternatively, use inbound filters to restrict which prefixes each peer can advertise.
11. Question: Your organization’s routing policies require you to advertise different prefixes to each ISP. How do you achieve this without duplicating configurations?
Answer: Use route-maps or policy-based advertisements. By tagging routes with different communities or attributes, you can control which routes are advertised to specific peers without maintaining separate configurations per ISP.
12. Question: Your BGP network is suffering from inconsistent routing between regions due to overlapping policies. How do you centralize and standardize control?
Answer: Implement a Route Policy Server (RPS) or use centralized BGP configuration management tools such as NetConf or Ansible. Standardized templates ensure consistent policy enforcement across routers and regions, minimizing configuration drift.
13. Question: You suspect a route hijacking attempt from a malicious AS. How would you verify and mitigate this?
Answer: Check the AS_PATH attribute for unexpected or fake AS numbers. Validate route origins using RPKI (Resource Public Key Infrastructure). You can also filter suspicious prefixes manually and report incidents to the affected Regional Internet Registry (RIR).
14. Question: After enabling RPKI validation, some legitimate prefixes are being dropped. What could be the reason?
Answer: This happens when ROA (Route Origin Authorization) entries are misconfigured or missing. Ensure that all legitimate prefixes are registered correctly in RPKI databases. Temporarily set the validation mode to “warn” instead of “drop” during troubleshooting.
15. Question: Your BGP network spans multiple data centers and cloud regions. How can you maintain consistent routing policies across on-prem and cloud networks?
Answer: Use BGP route reflectors or SDN controllers that integrate with cloud routing APIs. Apply the same route-maps and community tagging rules across all regions to ensure consistent traffic engineering and route control between data centers and clouds.
These advanced BGP scenarios demonstrate your ability to manage large-scale, distributed networks with precision. Strong answers show expertise in scalability, redundancy, route security, and operational stability — the key skills expected from senior network professionals.
Section 4: Scenario-Based BGP Configuration and Implementation Questions and Answers
This section highlights practical configuration scenarios that often appear in technical interviews. These questions test how you approach real BGP setups, apply routing policies, and ensure stable network performance.
1. Question: You are setting up eBGP between two routers from different ISPs. The routers are not directly connected but must still establish a BGP session. How would you configure it?
Answer: Since eBGP expects peers to be directly connected, you must enable ebgp-multihop to increase the TTL value. Configure the neighbour relationship using each router’s loopback interface and add the command neighbor [IP] ebgp-multihop [value]. Also, ensure the correct route exists to reach the remote peer’s loopback.
2. Question: You are tasked with advertising a default route to all downstream BGP peers. How can you achieve this?
Answer: Use the neighbor [IP] default-originate command to advertise a default route to peers. This dynamically injects a 0.0.0.0/0 route into BGP without needing to manually configure it in the routing table. Alternatively, you can use the network 0.0.0.0 mask 0.0.0.0 command if a default route exists locally.
3. Question: One of your internal routers cannot reach external networks even though eBGP is properly configured on edge routers. What configuration is missing?
Answer: The internal router likely needs iBGP sessions with the edge routers or a route reflector to learn external routes. Without iBGP propagation, internal devices cannot receive routes learned from eBGP peers, causing loss of connectivity to external destinations.
4. Question: You have multiple eBGP peers and want to advertise different prefixes to each one. How can you control which routes go to which peer?
Answer: Apply route-maps on outbound updates. Route-maps allow you to match specific prefixes and apply “permit” or “deny” conditions for each peer. You can also use prefix-lists with route-maps for more granular advertisement control. This ensures each ISP receives only relevant routes.
5. Question: A new customer requires you to advertise only their specific subnet (for example, 192.168.10.0/24) to external peers. What configuration would you use?
Answer: Use the network 192.168.10.0 mask 255.255.255.0 command under BGP configuration to advertise the subnet. Ensure the exact route exists in the routing table, as BGP will only advertise routes that are locally known. Optionally, apply a route-map to filter and ensure only customer-specific prefixes are shared externally.
These practical configuration scenarios test your ability to apply theoretical BGP knowledge in hands-on environments. They also reflect real troubleshooting and implementation challenges faced by network professionals in production setups.
How to Prepare for a Border Gateway Protocol (BGP) Interview?
If you’re preparing for a network engineering or routing-focused interview, a strong understanding of BGP (Border Gateway Protocol) is non-negotiable. BGP is the backbone of internet routing, and employers often use it to gauge your grasp of real-world networking, scalability, and troubleshooting.
Preparing for a BGP interview involves delving deeply into concepts, configuration, and problem-solving scenarios. The goal is not just to recall commands but to explain why BGP behaves the way it does under different routing conditions. Here’s a structured 2-week schedule to help you build solid BGP knowledge step-by-step.
| Phase | Focus Area | Key Topics to Cover | Suggested Activities | Outcome |
|---|---|---|---|---|
| Phase 1: Core Concepts (Day 1–2) | BGP Fundamentals | What is BGP, BGP vs IGPs, Autonomous Systems (AS), Path attributes | Study RFC 4271 overview, read Cisco or Juniper docs | Understand BGP’s role in interdomain routing |
| Phase 2: Configuration Basics (Day 3–4) | BGP Setup | eBGP vs iBGP, BGP neighbor relationships, Peer configuration | Configure basic BGP on Cisco Packet Tracer or GNS3 | Confidently establish BGP sessions between routers |
| Phase 3: Path Selection (Day 5–6) | BGP Decision Process | Attributes: Weight, Local Preference, AS_PATH, MED, Origin, Next Hop | Practice changing path selection using attributes | Learn how BGP chooses and manipulates best paths |
| Phase 4: Advanced Features (Day 7–8) | Optimization & Control | Route Reflectors, Confederations, Route Maps, Prefix Lists | Simulate large networks and apply policies | Gain control over routing behavior and scalability |
| Phase 5: Troubleshooting (Day 9–10) | Debugging & Stability | BGP flapping, Route dampening, Neighbor issues, Convergence time | Use show ip bgp, debug bgp, and logs | Become proficient in diagnosing routing problems |
| Phase 6: Security & Best Practices (Day 11–12) | BGP Security | Prefix filtering, TTL Security, MD5 Authentication, Route leaks | Review recent BGP hijack case studies | Learn how to secure and maintain BGP sessions |
| Phase 7: Interview Readiness (Day 13–14) | Practice & Review | Scenario-based questions, Real-world case studies | Practice with mock interviews and online labs | Confidently explain configurations and reasoning |
Expert Corner
BGP remains one of the most powerful and complex routing protocols in modern networking. From connecting global ISPs to managing enterprise multi-homed environments, its flexibility and policy-driven design make it indispensable for large-scale routing.
Preparing for BGP interviews requires more than memorizing commands — it demands a strong grasp of how BGP behaves in real-world scenarios. Understanding neighbour relationships, path selection, route filtering, and troubleshooting can help you stand out in both technical and practical discussions.
Whether you are a beginner learning the basics or an experienced engineer handling large-scale deployments, mastering scenario-based BGP questions will help you demonstrate analytical thinking and problem-solving skills that recruiters look for in network professionals.
