Top 25 BGP Interview Questions and Answers (2024)

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 came up with the top BGP interview questions and answers to help everyone with their interviews. If you are looking to move ahead in your career, you can also check out – OSPF BGP Training.

About 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.

Here are the top BGP Interview questions and answers. If you are looking for BGP Interview questions and answers PDF, then you can skip directly to the end of this blog.

Basic BGP Interview Questions and Answers

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

Q1. What is an Autonomous System (AS)?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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.

These are the top 10 BGP interview Questions and answers for freshers. Let’s move on to BGP interview Questions for experienced candidates.

Advanced BGP Interview Questions and Answers

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

Q11. How much Memory should my router have to get my ISP’s whole BGP routing table?

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.

Q12. What is the BGP AD value?

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:

Q13. What is the difference between BGP and RIP?

Following are the basic differences between BGP and RIP:

Q14. What is the basic difference between EIGRP and BGP?

Here are a few differences between EIGRP and BGP:

Q15. What is BGP MED, and how does it work?

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.

Q16. What are communities in BGP, and what are their different types?

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

Q17. Explain the use of BGP Confederations for scalability.

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.

Q18. What tools and techniques can be used to monitor BGP performance?

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.

Q19. How can BGP be used for traffic engineering in a network?

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.

Q20. Why AS numbering and assignment is important?

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.

These are the advanced BGP Interview Questions and Answers for experienced network engineers.

Scenario-Based BGP Interview Questions and Answers

Q21. 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?

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.

Q22. 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?

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.

Q23. 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?

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.

Q24. Your organization is experiencing frequent route leaks from a BGP peer, causing unwanted traffic. How would you approach the problem?

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.

Q25. 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?

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.

For more questions, you can download the PDF.

BGP Interview Questions and Answers PDF

If you are looking for a PDF with more BGP interview questions and answers, or a complete list of CCNA Interview questions and answers then visit this blog –

CCNA Interview questions and answers

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.

BGP is an important part of our CCNA course, if you focus on learning CCNA. We would love to hear from you, so do comment your feedback/suggestions.