Top 50 BGP Interview Questions and Answers (Latest – 2026)

Introduction

BGP stands for Border Gateway Protocol. It is a common EGP (exterior gateway protocol) that is designed to transfer information on reachability and routing between autonomous systems (AS) via the Internet. This protocol is one of the favourite topics of interviewers.

Hence, we have compiled the top 50 BGP Interview Questions and Answers to help freshers and experienced professionals prepare for networking interviews. This guide covers BGP fundamentals, path attributes, troubleshooting, route reflectors, and real-world scenarios commonly asked in interviews. If you want to gain hands-on experience with BGP and enterprise routing, you can also explore our BGP and OSPF Training Course at PyNet Labs.

What is BGP (Border Gateway Protocol)

The Border Gateway Protocol (BGP) is the routing protocol for exchanging routing information throughout the Internet. It enables Internet Service Providers (ISP) to establish interconnections with one another, thereby allowing end-users to access multiple ISPs. BGP is the only protocol designed to handle a network of the Internet’s size and to manage multiple connections to separate routing domains effectively.

BGP is used in data center networks, corporate networks, and service provider networks to communicate routing data between different locations or regions by choosing the best optimal path, which further depends on various factors such as distance, bandwidth, reliability, and policies.

BGP Interview Questions and Answers for Freshers

Here are the top BGP Interview Questions and Answers for freshers:

Q1. What is an Autonomous System (AS)?

Answer – An Autonomous System (AS) is a collection of IP networks and routers, which are under the control of a single organization. This Organization presents a common routing policy to the internet. Each AS is assigned an unique number which is known as ASN or Autonomous System Number. BGP uses these ASNs to identify different networks where data needs to travel.

Q2. What are the features of BGP?

Answer – Some of the best features of BGP are as follows:

• It is Open standard Path-vector protocol.
• It is a classless routing protocol.
• It uses the Path vector algorithm.
• Administrative distance for eBGP is 20, iBGP-200
• It is employed to exchange router information between autonomous system.
• The hello timer for BGP is 60 sec whereas the hold on timer is 180 sec.
• TCP port number 179 is used by BGP.

Q3. What is the difference between eBGP & iBGP neighbors?

Answer – The major difference between eBGP and iBGP neighbors is:

• The routers within the same AS (autonomous system) forms an iBGP neighborship.
• The routers in different AS (autonomous system) forms an eBGP neighborship.

Q4. Can Router on different subnet become BGP Neighbors?

Answer – It is not an essential condition for BGP that neighbors need to be attached to the same subnet. In fact, a TCP connection between the routers is applied by BGP to pass the BGP message. This allows routers to be anywhere.

So, yes routers on different subnets can become BGP neighbors.

Q5. What is the Loop prevention mechanism in BGP?

Answer – Two mechanisms are used by BGP to prevent loops:

1. When an iBGP peer provides a route to a router, the same route is not shared with any another iBGP peer by that router.

2. Using AS_PATH – A BGP router adds its own ASN to the AS_PATH when it advertises to an eBGP peer. A route gets ignored if BGP router receives an update where router advertisement lists it an AS_PATH with its own ASN.

Note: ASN is not added by the BGP router when it is advertising to an iBGP peer.

Q6. What is the difference between the hard reset and soft rest in BGP?

Answer – Here is a comparison to show the difference between hard reset and soft reset in BGP –

Hard Reset – The local router, in case of hard reset will bring down:

• The neighborship
• The underlying TCP Connection
• All the BGP table

In hard reset, it also removes the entire learning from the neighbor and the command used for hard reset is #clear ip bgp.

Soft Reset – In a soft reset event, the router will not drop the BGP neighborship or the TCP connection. In this reset, the local router will resend all the outgoing updates and processes the incoming updates again.

It will also readjust the BGP table on the current configuration. The command used for soft reset is #clear ip bgp * soft.

Q7. What are different BGP Message Types?

Answer – Different BGP message types are as follows:

• Open – Open message is utilized to establish a neighbor connection and to establish exchange parameter, such as the autonomous system number as well as the authentication value.
• Keepalive – These messages are periodically sent (every 60 seconds, by default) to make sure that the remote server is in operation. If a router doesn’t receive a KEEPALIVE from the peer for a Hold-time interval (by default, it is 180 seconds) the router will announce that the peer is dead.
• Update – It is used to exchange Path Attributes as well as the length/prefix (NLRI) which use these characteristics.
• Notification – It is utilized to inform of BGP error. This causes a reset of the neighbour relationship.

Q8. What are the different states of BGP?

Answer – Here are the various states of BGP:

• Idle – It is the initial state of BGP.
• Connect – If the TCP connection gets completed, the BGP process will continue to the Open Sent State whereas, if it fails, it will go to the active state.
• Active – BGP will again try TCP three-way handshake in order to establish a connection with remote BGP neighbor. It will move to the Open Sent State if it is successful.
• Open sent – When the BGP has established the TCP connection and has sent an OPEN Message. It awaits a reply of the OPEN Message. If it receives the message, a KEEPALIVE message will be sent by the BGP peer.
• Open confirm – BGP waits for a reply of the KEEPALIVE message.
• Established – At this state, all the neighbor parameters have matches, the relationship between neighbors have been established and update message can now be exchanged between the peers.

Q9. Explain BGP Path Attributes?

Answer – Various attributes are used by BGP to determine the best path to a destination.

1. Next Hop – It is used to list the next-hop IP address that can be used to reach the prefix. The router can’t use this router if no route is available to reach the next-hop.

2. Weight – It is used to give advantage to the path with the highest weight. It is locally significant to the router and is Cisco proprietary.

3. Local Preference – It is used to give advantage to the path with highest local preference. It is used within an AS and exchanged between iBGP routers. Default is set to 100.

4. Locally injected routes – Routes that are injected using network command are called Locally injected routes. These routes are better than iBGP/eBGP learned.

5. AS Path – It is applied to outbound routes. It helps in deciding the best inbound path. With the AS-Path attribute, two things can be accomplished that are: prepend or filter. Smaller is preferred.

6. Origin – This attribute is used to identify the originating source of the route. The origin codes are as follows (listed in order of preference for route selection):

• i (IGP) – It is emerged from an IGP (interior gateway protocol), such as OSPF. This usually indicates that network command under the BGP process was used to inject the route into BGP. An origin code that has “i” is most desired.
• e (EGP) – It is originated from an EGP (external gateway protocol).
• ? (incomplete) – It means the origin of it is Unknown. This specifies that the route was redistributed into BGP from either connected, static, or IGP routes. An origin code that has “?” is the least desired.

7. Multi-Exit Discriminator (MED) – This attribute is applied to the outbound routes. It finds the best inbound path into the AS. When viewing the BGP routing table, MED is seen as the BGP metric.

By default, the MED value is 0 and a smaller/lower value is preferred.

8. Neighbour type – In this, the eBGP is preferred over iBGP

9. IGP metric – With this attribute, the route with nearest IGP neighbour (lowest IGP metric) is preferred.

10.  eBGP route – The Oldest route is preferred.

11. Neighbour Router ID – Lowest is preferred.

12. Neighbour IP Address – Lowest is preferred.

Q10. What is BGP Local preference?

Answer – The BGP local preference attribute is the second attribute. It is used in an AS to select the exit route from a local perspective. By default, its value is set to 100 and the path having highest local preference becomes more desired. This attribute is not shared between routers.

Q11. What is the difference between an Autonomous System Number (ASN) and an IP Address?

Answer – An IP address identifies a specific device or network interface within a network, whereas an Autonomous System Number (ASN) identifies an entire network or organization participating in BGP routing.

For example, a router may have an IP address of 192.168.1.1, while the organization operating that router may own ASN 65001.

BGP uses ASNs to exchange routing information between different autonomous systems and to help prevent routing loops through the AS Path attribute.

Q12. Why is BGP called a Path Vector Routing Protocol?

Answer – BGP is called a Path Vector Routing Protocol because it advertises the complete AS path that a route has travelled.

Unlike distance-vector protocols that only advertise metrics such as hop count, BGP includes the list of autonomous systems through which the route has passed. It allows routers to make smarter routing decisions and prevent routing loops by checking whether their own AS number already exists in the received AS Path.

Q13. What is the BGP Routing Table?

Answer – The BGP Routing table is a BGP Database where it stores all the routes learned from BGP Peers.

There may be multiple routes to a given destination in the BGP Table. BGP uses its path selection process to find the best path and installs this path in the routing table. There may be a lot of paths available in the BGP table compared to the BGP Routing table, as not every path is selected as the best path.

Q14. What is a BGP Peer?

Answer – A BGP Peer, also known as a BGP neighbor, is a router with which a BGP router establishes a session to exchange routing information. Routes can only be exchanged once both routers have established a TCP connection on port 179 and successfully completed the BGP neighbor relationship process.

There are two types of BGP Neighbors:

• eBGP neighbors (different AS numbers)

• iBGP neighbors (same AS number)

Without a neighbor relationship, it is not possible to exchange BGP routes.

Q15. Which Transport Layer Protocol does BGP use and why?

Answer – BGP uses TCP protocol as its transport protocol and operates on TCP port 179. TCP provides:

• Reliable delivery

• Packet sequencing

• Error detection

• Flow control

Using TCP allows BGP to focus on route exchange and policy decisions while TCP handles reliable communication between peers.

These are the top 15 BGP interview Questions and answers for freshers. Let’s move on to BGP interview Questions for experienced candidates.

BGP Interview Questions and Answers for Experienced

Here are a few important BGP Interview Questions for experienced network engineers:

Q16. How much Memory should my router have to get my ISP’s whole BGP routing table?

Answer – Several factors, such as the BGP features, router, route dampening, the number of paths established, the number of available alternative paths, and VPN settings, affect the amount of Memory required to store BGP routes. Without this information, we can’t accurately estimate how much RAM is required to store a certain number of BGP routes.

Cisco recommends that your router must have at least 512 MB of RAM to store a BGP routing table from a single BGP peer. However, knowing how to preserve memory and achieve efficient routing without acquiring the complete Internet routing table is crucial.

Q17. What is the BGP AD value?

Answer – Administrative Distance (AD) is a metric that may be used to determine the reliability of the routing information collected from an adjacent router. Routers give each source an AD value that falls anywhere from 0 to 255. A lower number is more reliable within this range than a larger one.

After determining the path with the smallest value of AD, the next step will be to add that path to the routing table to arrive at the most efficient path for connecting to the distant target network.

Below is the chart that best describes the default AD values by Cisco:

Route Sources	Default AD
Connected Interface	0
Static Route	1
External BGP	20
EIGRP	90
OSPF	110
RIP	120
External EIGRP	170
Internal BGP	200
Unknown	255 (Not used route)

Q18. What is the difference between BGP and RIP?

Answer – Following are the basic differences between BGP and RIP:

Border Gateway Protocol (BGP)	Routing Information Protocol (RIP)
BGP is based on the best-path algorithm.	RIP is based on the Bellman-Ford algorithm.
BGP is designed for large networks that span multiple ASes, and it can handle thousands of routes.	RIP is suitable for smaller networks that have less than 15 hops.
BGP is a hybrid protocol that combines features of distance vector and link state protocols. It uses attributes to describe the properties of each route and applies policies to manipulate them.	RIP is a pure distance vector protocol that does not use attributes or policies.
BGP has a fully meshed topology where every router has a direct connection with every other router in the same AS.	RIP has a flat network topology where routers only communicate with their directly connected neighbors.

Q19. What is the basic difference between EIGRP and BGP?

Answer – Here are a few differences between EIGRP and BGP:

Enhanced Interior Gateway Routing Protocol (EIGRP)	Border Gateway Protocol (BGP)
EIGRP uses a Dual Distance Vector algorithm.	BGP uses the Best-path algorithm.
EIGRP is a hybrid protocol.	BGP is a Vector state protocol.
EIGRP is for large companies or organizations.	As compared to EIGRP, BGP is relatively used for very large organizations.
EIGRP is for internal gateways.	BGP is basically used for connecting to the external gateways.

Q20. What is BGP MED, and how does it work?

Answer – With MED, you can manage how other autonomous systems connect to yours so that they may access a specific prefix.

BGP MED is only propagated to neighboring autonomous systems as a network property. The more probable the route is to be taken, the lower the MED.

Q21. What are communities in BGP, and what are their different types?

Answer – In the case of BGP, the concept of a community is a method by which routes may be categorized. They are like tags that tell routers what to do. They are used to determine the routes that have something in common, such as network or region. They can be used to restrict routes, define routing policies, or even determine the manner in which traffic is routed. It is worthwhile to regard communities as tags that are applicable to routes. Routers can employ these tags to decide whether they want to take a particular route or not to take a particular route. This makes the networks work efficiently and assists in administering certain routes.

Different types of communities used in BGP are:

• No-Advertise Community
• Community with No Exports
• Local AS Community
• Graceful Shutdown Community
• Extended Community
• Route Target Community
• Route Origin Community

Q22. Explain the use of BGP Confederations for scalability.

Answer – BGP Confederations assist in making a network more scalable. Consider a vast network of routers and switches before you. It is hard to manage. Confederations further partition the network into various segments known as sub-autonomous systems. They have their own routers and network systems in each group.

These groups can also communicate with each other through BGP. This will, in turn, make the network more scalable and also easy to manage. It is kind of like handling a big puzzle where instead of taking the big puzzle at once, you take parts of it that you can easily handle.

With BGP confederations, each piece can address the others, and, at the same time, they are not overloaded with the network. In this way, new routers and groups, or we can say more and more nodes, are added to the network, and it becomes less difficult to incorporate them into the network.

Q23. What tools and techniques can be used to monitor BGP performance?

Answer – To monitor BGP performance, you can use tools like:

• Router logs: Check router logs to see what is happening with BGP.
• Network management systems: Tools like Nagios or SolarWinds can monitor BGP performance.
• BGP monitoring software: Specialized software like BGPlay or BGPmon can track BGP updates and issues.
• Ping and traceroute: Simple tools to test connectivity and routing.
• Route servers: These can help you see what routes are being advertised and learned.

Q24. How can BGP be used for traffic engineering in a network?

Answer – BGP can be used for traffic engineering by:

• Setting up multiple paths: BGP can advertise multiple paths to a destination, allowing traffic to be split between them.
• Using route maps: Route maps can be used to manipulate BGP routes and influence traffic flow.
• Setting up route reflectors: Route reflectors can help control the flow of traffic between different parts of the network.
• Using communities: Communities can be used to tag routes and influence how traffic is routed.
• Using AS path prepending: This can be used to influence the path that traffic takes.

Q25. Why AS numbering and assignment is important?

Answer – AS or Autonomous System numbering and assignment are critical factors within BGP because the routers have to be able to differentiate one from the other and route the traffic correctly. Each AS is provided with a unique number, which is analogous to an identity tag that assists routers in the determination of the direction of the flow of traffic.

Some consequences of the absence of unique AS numbers are that routers can mistake routing paths and can deliver traffic to the wrong location. This may lead to issues like traffic looping or dropping of packets in the network. The AS number also assists in the avoidance of routing loops and routing of the traffic. It is like an address that aids the routers in reaching the traffic at its proper location.

Q26. Explain the BGP Best Path Selection Process. 

Answer – When there are multiple routes to the same destination, BGP uses its path selection algorithm to choose the best route. Here are some of the common attributes: 

1. Highest Weight (Cisco-specific) 

2. Highest Local Preference 

3. Locally originated route 

4. Shortest AS Path 

5. Lowest Origin Type 

6. Lowest MED 

7. eBGP over iBGP 

8. Lowest IGP metric to NEXT_HOP 

9. Oldest path 

10. Lowest Router ID 

BGP does not always choose the shortest available path to the destination; it selects the route with the most preferred attributes according to the path selection process. 

Q27. What is a Route Reflector and why is it used? 

Answer – A route reflector is a BGP router that allows iBGP routes to be redistributed to other iBGP peers without requiring a full mesh. Every iBGP router must peer with every other iBGP router, but as the network grows, it becomes difficult to manage. 

A route reflector solves this scalability problem by acting as a central point that reflects routes between clients. This reduces the number of BGP sessions required in large networks. 

Q28. What is BGP Route Dampening? 

Answer – Route Dampening is a mechanism used in BGP to reduce the impact of unstable routes that frequently flap between reachable and unreachable states. BGP assigns penalty points to any route that repeatedly changes its state. If the penalty becomes more than the configured value, that route gets temporarily suppressed. 

When the route becomes stable again, the penalty decreases, and the route becomes available again. Route dampening is a great way to improve network stability, but it should be configured very carefully because it can delay legitimate route advertisements. 

Q29. What is the NEXT_HOP Attribute and why is it important? 

Answer – The NEXT_HOP Attribute is used to find the IP address that should be used to reach a destination network. BGP may learn routes successfully but fail to install them into the routing table if the NEXT_HOP address is unreachable. 

This is very common in iBGP environments. Network engineers use the command “next-hop-self” to make sure internal routers can reach the advertised next hop. 

This attribute is one of the most important attributes to verify when troubleshooting route reachability issues. 

Q30. What is BGP Route Aggregation? 

Answer – BGP Route aggregation is the process of combining multiple network prefixes into a single summarized route. For example: 

192.168.0.0/24 

192.168.1.0/24 

192.168.2.0/24 

192.168.3.0/24 

Can be summarized as: 

192.168.0.0/22 

Some of the benefits of Route aggregation are: 

• Smaller routing tables 

• Reduced memory usage 

• Faster convergence 

• Lower routing overhead 

Route aggregation is widely used by ISPs and large enterprises. 

These are the advanced BGP Interview Questions and Answers for experienced network engineers. Let’s see some scenario-based Interview questions related to BGP.

Scenario-Based BGP Interview Questions and Answers

Here are some scenario-based interview questions related to BGP:

Q31. Two companies are merging, each having their own BGP setup and AS. What challenges might arise during integration of two networks and how to solve them?

Answer – The primary challenge that needs to be avoided is creation of routing loops and ensuring consistent policies between the two networks. This can be done by:

• Merge both AS into one AS by decommissioning one of the AS numbers. However, this can be difficult if there are a lot of dependencies on AS paths.
• Alternatively, you can use BGP Confederation or AS_Path filtering to maintain separate AS while integrating their routing.

Another issue that could arise is conflicting policies. This can be addressed by creating unified routing policies that take into account existing differences in Local Preference, MED, etc.

Q32. A network admin reports that BGP is up, but some routes from a BGP peer are missing from the routing table. What non-technical factors could contribute to this?

Answer – Here are some non-technical factors that can contribute to this:

• Routing Policies: Organizational policies might dictate that certain routes will not be accepted or advertised.
• Agreements: Agreements between Autonomous Systems might restrict specific routes. It might happen as both parties require to adhere to pre-agreed policies.
• Miscommunication: Communication issues between teams might lead to misunderstanding on how routes are supposed to be handled. It could lead to incorrect filtering or policy application.

Q33. You are receiving routes from a BGP peer, but you notice that traffic is not flowing as expected. How would you verify that the NEXT_HOP attribute is correct and that peer is reachable?

Answer – The NEXT_HOP issues can be verified without even touching configuration.

• First, we should check the IP Addresses advertised in the BGP routes and make sure that the NEXT_HOP IP is reachable in your network. If this router is not reachable, BGP won’t use this route.
• Discuss with the network admin to ensure that the IGP in your network is advertising the correct next-hop for these routes.
• At last, review any route filtering policy that might be blocking the visibility of next-hop router.

Q34. Your organization is experiencing frequent route leaks from a BGP peer, causing unwanted traffic. How would you approach the problem?

Answer – To address this issue:

• Policy Review: Firstly, you should request the BGP peer to review their outbound routing policies to ensure they are not advertising more that what is allowed.
• Route Filtering: Implement strict filters to prevent unauthorized routes from being accepted.
• BGP Communities: You can work with the peer to use the BGP Communities to filter routes properly. It will ensure that only the correct routes are advertised.
• Communication: At last, you should establish clear lines of communication with the peer. It will help you address any future routes leaks.

Q35. Your company wants to migrate its BGP-based WAN to a new data center. What planning steps should be taken before initiating the migration to ensure a smooth transition?

Answer – Here are some planning steps you should take:

• Capacity: First we have to ensure that the new data center network has enough capacity to handle the BGP session and route advertisements required by the WAN.
• Redundancy: You should implement multiple BGP paths during the migration to prevent downtime. After that, you can gradually shift traffic to the new data center while keeping the old one active.
• Routing Policy Review: You should review the routing policy and implement them properly in the new environment too.
• Testing: Conduct some tests before migration, including connectivity issues. It will ensure that the new Data Center can handle the incoming/outgoing traffic.
• Communication: At last, you should coordinate with all stakeholders and peers to make them aware of the migration and tis timeline.

Q36. Your company connects to two ISPs. Management wants Internet traffic to primarily use ISP-A while ISP-B acts as backup. How would you achieve this using BGP? 

Answer – The best way is to manipulate BGP Path selection attributes to make ISP-A more attractive. Work on local preference because it influences outbound path selection throughout the autonomous system. 

That way, routes that are learned from ISP-A will get a higher Local Preference than routes learned from ISP-B. 

In any case where ISP-A fails, BGP will automatically select routes through ISP-B. This is the best design possible as it provides primary and backup Internet connectivity without manual intervention. 

Q37. A branch office reports intermittent connectivity to cloud applications. The BGP session remains established throughout the incident. How would you investigate? 

Answer –This problem cannot be related to neighbor formation as the BGP session remains established. So, you should check for: 

• Route changes 

• Path selection behavior 

• Packet loss 

• Latency 

• ISP performance 

• Route flapping 

Compare the routing table before and during the issue to find out whether the traffic is being redirected through a less optimal path. Your aim should be to find out whether connectivity problems are caused by routing instability or transport degradation. 

Q38. After adding a new ISP connection, users report that outbound traffic follows the correct path, but return traffic arrives through a different ISP. Why is this happening? 

Answer – This is a clear case of asymmetric routing. As we know, the outbound traffic selection is controlled by the local AS, but inbound traffic selection is controlled by external networks. 

To influence inbound traffic, you should use: 

• AS Path Prepending 

• MED 

• Provider-specific communities 

Using these techniques, you can encourage external networks to prefer one ISP over another. The solution depends on understanding that inbound and outbound traffic engineering are controlled differently in BGP. 

Q39. Your organization acquires another company that uses a different AS number. What factors should be considered before connecting the two BGP environments? 

Answer – These are some of the most important factors that should be considered: 

• Route overlap 

• AS number conflicts 

• Routing policies 

• Route filtering 

• Security requirements 

• Traffic engineering requirements 

You should also check if the organizations should maintain separate AS numbers, use confederations, or you can migrate them into a single AS. You should plan everything very carefully to avoid any route leaks and unintended traffic flows during integration. 

Q40. A company wants to advertise only a summarized route to its ISP while maintaining visibility of individual routes internally. How would you design the solution?

Answer – The best solution would be to implement route summarization at the edge BGP routers. This way:

• Internally, routers would continue exchanging detailed routes using BGP or an IGP.

• Externally, only the summarized prefix would be advertised to the ISP.

This reduces the size of the routing table, improves scalability, and minimizes routing updates while saving internal routing granularity.

BGP Troubleshooting Interview Questions and Answers 

Q41. Two BGP neighbors are configured correctly but remain stuck in the Active state. How would you troubleshoot and resolve the issue?

Answer – The active states mean that the router is attempting to establish a TCP connection but cannot successfully form a BGP session. You should start by verifying IP connectivity between the neighbors using ping and traceroute.

Since BGP relies on TCP port 179, confirm that no ACLs or firewalls are blocking the connection. After that, you should verify the configured neighbor IP addresses and AS numbers. A common issue is configuring the wrong remote AS.

If the peers are using loopback interfaces, make sure the update-source command is configured correctly and that routes to the loopback addresses exist. After correcting the issue, clear the BGP session and verify that the neighbor transitions from Active to Established.

Q42. A BGP session is Established, but no routes are being received from the neighbor. How would you troubleshoot and fix the issue?

Answer – If the session is established, then TCP connectivity and neighbor formation are working. You should start by examining route advertisements from the peer. Verify whether the neighbor is actually advertising routes and whether route filters such as prefix lists, route maps, or distribute lists are blocking them.

You should also make sure that the routes exist in the peer’s BGP table. If you find that a filtering policy is preventing routes from being accepted, modify the policy and re-advertise the routes.

Finally, confirm that the routes appear in the BGP table and are eligible for installation in the routing table.

Q43. What would you do if routes were visible in the BGP table but not in the routing table?

Answer – When routes appear in the BGP table but not the routing table, BGP has learned the route but has not selected it for installation. You should check whether another routing protocol offers better administrative distance.

Next, verify that the NEXT_HOP attribute is reachable because an unreachable next hop is one of the most common causes of this issue. If the next hop is unreachable, I would either fix the underlying routing issue or configure next-hop-self where appropriate.

After resolving the reachability issue, you should verify that the route becomes the best path and appears in the routing table.

Q44. After adding a new ISP connection, Internet traffic continues using the old ISP. How would you troubleshoot and correct the issue?

Answer – You should start by checking the BGP best-path selection process. The most likely cause is that path attributes such as Local Preference, Weight, AS Path, or MED still favor the original ISP. Examine the received routes and compare their attributes.

If the business requirement is to prefer the new ISP, you should adjust the Local Preference because it is one of the most effective attributes for influencing outbound traffic.

Once you have applied the changes, you should verify that the routing table selects the desired path. After that, you should confirm traffic flow using traceroute.

Q45. A route is being advertised to an external BGP peer when it should not be. How would you troubleshoot and resolve the issue?

Answer – This kind of issue mainly points towards a route filtering problem. First, you should find which route is being leaked and determine where it originated. Next, review outbound route maps, prefix lists, and community policies applied to the neighbor.

If the enterprise doesn’t want the route to be advertised externally, you should update the filtering policy to block it. After making the changes, verify if the route is still advertised externally and confirm with the external peer that the route has been withdrawn.

Q46. BGP route suddenly disappears after a network change. How would you troubleshoot and restore connectivity?

Answer – First, you need to find out if the route is really withdrawn or just filtered. You should compare the current BGP table with previous routing information and find out when the route disappears. Then, you should check:

• Prefix-list changes

• Route-map changes

• Neighbor configuration changes

• Network statement modifications

Once you find out which configuration caused the withdrawal, restore the missing advertisement or correct the filtering policy and verify that the route reappears in both the BGP table and routing table.

Q47. How would you troubleshoot and fix the issue when BGP routes are flapping repeatedly and causing instability?

Answer – Route flapping usually indicates an unstable network, interface, or peer. First, you should find out which route is flapping and what the reason for the flapping is, whether the issue is related to a physical link problem, routing instability, or frequent configuration changes.

If the interface is unstable, you should fix the physical issue before making any BGP changes.

If frequent route advertisements are causing instability, then implement route dampening to suppress excessively flapping routes.

After stabilizing the problem, you should monitor the BGP session to make sure that route advertisements remain consistent.

Q48. An iBGP router receives routes but does not advertise them to another iBGP router. How would you resolve this issue?

Answer – iBGP routers do not advertise routes learned from one iBGP peer to another iBGP peer, so this behavior is expected. To solve this issue, we should verify the network design.

If the environment is small, establish a full mesh of iBGP sessions.

However, if the network is large, implement Route Reflectors or Confederations to improve scalability.

After implementing the solutions, verify that routes are successfully propagated throughout the autonomous system.

Q49. After configuring a new neighbor, the BGP session keeps resetting every few minutes. How would you troubleshoot?

Answer – This type of frequent session resets show instability in the underlying connection or a configuration mismatch. So, you should examine:

• Interface stability

• CPU utilization

• BGP keepalive and hold timers

• Authentication settings

• Network reachability

Once authentication is configured, verify that both peers use the same password. After that, monitor the session uptime and confirm that the neighbor remains stable for an extended period.

Q50. How would you troubleshoot and correct the issue if a router receives multiple routes to the same destination, but BGP selects an unexpected path?

Answer – You should first analyze the route selection process attribute by attribute. Compare these metrics:

• Weight

• Local Preference

• AS Path Length

• Origin Type

• MED

• eBGP vs iBGP preference

These metrics will allow you to understand why BGP selected this unexpected path. If the selected path does not align with business requirements, adjust the appropriate attribute, such as Local Preference for outbound traffic engineering or AS Path Prepending for influencing inbound traffic.

After making the changes, check if BGP is now selecting the intended route and confirm traffic flow using routing table and traceroute outputs.

BGP Interview Preparation Tips

Preparing for a BGP interview requires more than just memorizing definitions. Interviewers often focus on real-world troubleshooting, routing decisions, and practical networking scenarios.

Here are a few tips to help you prepare:

• Understand the difference between eBGP and iBGP.
• Learn how BGP Path Attributes influence route selection.
• Practice the BGP Best Path Selection process.
• Be familiar with Route Reflectors and BGP Confederations.
• Understand route filtering using Prefix Lists and Route Maps.
• Learn how to troubleshoot common BGP issues such as Active state, route flapping, and unreachable NEXT_HOP.
• Practice scenario-based questions related to ISP connectivity, traffic engineering, and route leaks.
• Build hands-on BGP labs using Cisco Packet Tracer, EVE-NG, or GNS3 to gain practical experience.

A strong understanding of BGP fundamentals, troubleshooting techniques, and real-world routing scenarios will significantly improve your chances of clearing networking interviews.

Also If you’re preparing for networking interviews, you may also find these interview guides helpful:

• CCNA Interview Questions and Answers
• OSPF Interview Questions and Answers
• Networking Interview Questions and Answers
• TCP/IP Interview Questions and Answers

Or Visit Our Interview Quetsions HUB where you can find all Types of Interview Questions related to IT, Networking, Automation, Cyber security and more… – Interview Questions Hub

These guides cover routing, switching, networking fundamentals, and protocol-related questions commonly asked in networking and network engineer interviews.

This BGP Interview Questions and Answers guide has been reviewed by Chirag Dhall (CCIE #68677), Corporate Trainer at PyNet Labs, who has trained hundreds of networking professionals in technologies such as BGP, OSPF, MPLS, CCNA, CCNP Enterprise, SD-WAN, and Network Automation.

His industry experience and hands-on training expertise help ensure that the concepts and interview questions covered in this guide reflect real-world networking scenarios and current industry expectations.

Conclusion

In this blog post, we have discussed the commonly asked BGP interview questions and answers. We hope that this post will help you to prepare for your upcoming BGP interview and also helps in understanding some of the concepts and scenarios of BGP.