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BGP Interview Questions and Answers
Frequently Asked Questions
What are the drawbacks to BGP?
What is the significance of BGP?
Where is BGP routing most commonly used?
Last Updated: Jun 12, 2024

Top 30 BGP Interview Questions and Answers

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18 Jun, 2024 @ 01:30 PM


Every minute, a lot happens online. People are sending and receiving 333.2 billion emails each day. Do you know how your emails go to their target receiver? How do these data packets make their way over the internet? 

All of this is accomplished through Internet routing, and BGP (Border Gateway Protocol) is a routing protocol used to conduct Internet routing.

BGP is one of the favourite topics in CCNP interviews. This article will discuss the top 30 frequently asked BGP interview questions and answers. So, Without any further ado! Let’s start.

Also Read About, Pandas Interview Questions

BGP Interview Questions and Answers

  1. What exactly is BGP? What is the session type of BGP?

    Ans: Border Gateway Protocol (BGP) is a standardised exterior gateway protocol that allows autonomous systems (AS) on the Internet to share routing and accessibility information.

    It is classified as a Path vector Protocol. The Border Gateway Protocol is engaged in core routing decisions.
    It determines routing decisions depending on pathways, network regulations, or rule sets a network administrator provides.

    BGP has two session types:
    a.) Internal BGP(iBGP)
    b.) External BGP(eBGP)

    BGP can be utilised for routing within an autonomous system. The interior Border Gateway Protocol, often known as Internal BGP or iBGP, is used for routing within an autonomous system.
    The protocol's Internet implementation is known as Exterior Border Gateway Protocol, External BGP, or eBGP.

  2. What are some critical differences between iBGP and eBGP?

    Ans: The main differences between iBGP and eBGP are as follows:
  3. What do you mean when you mention "peers" in BGP?

    Ans: Peers are any two routers that have established a TCP connection to exchange BGP routing information in BGP. BGP neighbours are also known as peers.

  4. Is it possible for routers on different subnets to become BGP neighbours?

    Ans: BGP is frequently set up between two directly connected routers with other autonomous systems. 

    BGP routers don't require their neighbours to be on the same subnet. Instead, they employ a TCP connection between the routers to send and receive BGP messages, allowing neighbouring routers to be on the same or distinct subnets.

  5. What is the border gateway protocol's port number?

    Ans: The Transmission Control Protocol (TCP) port 179 is used by the Border Gateway Protocol.
  6. How can we determine the status of the BGP?

    Ans: There are two methods for deciding BGP status:
    a.)  In CONFIGURATION ROUTER BGP mode, type show config to see the BGP configuration.
    b.) In EXEC Privilege mode, run the show IP BGP summary command to see the BGP status.

  7. In the context of BGP, what do you mean by split-horizon?

    Ans: The split horizon is a method employed by distance vector protocols to avoid network routing loops. The basic concept is straightforward that is the information should never be routed back in the same direction from which it originated.

    Because distance vector protocols like Routing Information Protocol (RIP) are prone to routing loops, which occur when a data packet is repeatedly routed in an endless loop and routed through the same routers, a split-horizon is required. Split horizon is a protocol loop avoidance approach

  8. What do you mean by poison reverse in the context of BGP?

    Ans: The Poison Reverse method is a commonly used distance-vector routing approach. To solve the count-to-infinity problem, Poison Reverse is utilised. In other words, poison reverse is the inverse of the split horizon. Route advertising that would be suppressed by split horizon is instead advertised at an infinite distance with poison reverse. The RIP(Routing Information Protocol) is the basis for Poison Reverse.

  9. In the context of networking, what do you mean by Routing Information Protocol (RIP)?

    Ans: The Routing Information System (RIP) is a dynamic routing protocol that uses hop count as a routing statistic to discover the best path between source and destination networks. It is a distance-vector routing protocol that runs at the OSI application layer. Port 520 is used by the RIP protocol. 

    The hop count is the number of routers that connect the source and destination networks. The shortest path to a network is considered the best route and is thus placed into the routing table. RIP limits the number of hops permitted between source and destination, effectively eliminating loops.

    The maximum number of hops allowed by RIP is 15, and the network considers a hop count of 16 to be unreachable.

  10. What is a route reflector, and why does it need to be used?

    Ans: A route reflector is a BGP router permitted to ignore the iBGP loop avoidance rule. Under specific settings, route reflectors can broadcast updates received from an iBGP peer to another iBGP peer.
    Route reflectors bypass the complete mesh requirement, allowing for creating iBGP networks that scale simply and cleanly.

  11. What is BGP recursive lookup, and how does it work?

    Ans: The next-hop attribute of BGP recursive route lookup can be used to find a path to a network that the IGP is aware of. The Border Gateway Protocol would not work without the recursive lookup because it is constructed on top of recursive routing.

  12. What is an autonomous system number (AS), and how can You get one?

    Ans: AS numbers are globally unique numbers used to identify ASes and allow them to share outside routing information with their neighbours. An AS is a network of IP networks linked together and follows a single, well-defined routing strategy.

    There are a limited number of AS numbers available. As a result, it's critical to determine which sites require unique AS numbers and which don't. Sites that don't need their own AS number should utilise one of the AS numbers intended for private use, ranging from 64512 to 65535. Go to the AS Number Registration Services website to receive an AS number.

  13. What does the next hop of mean in the show IP BGP command output?

    Ans: This is one of the most frequently asked BGP interview questions and answers.
    A network in the BGP table with a next-hop address of indicates that the network was created locally through IGP redistribution into BGP or by a network or aggregation command in the BGP setup.

  14. What do the various BGP Path Attributes mean?

    Ans: BGP includes several Path Attributes that can be used to compare competing BGP paths (routes) in the BGP table to determine the best path (route).

    The following are some BGP Path Attributes:
    → Next Hop: Path Attributes display the Following Hop, which is the IP address of the prefix's following hop. It determines whether the Next Hop can be accomplished or not. The router does not use this route if no other route can reach Next Hop.

    → The weight Path Attributes is a numeric number that a router gives you to affect the route for a prefix when you obtain updates. It isn't well recognised among BGP colleagues that greater weights are preferred.

    → Local Preference is a collection of quantitative values as well. It's sent within a single autonomous system so that all routers in that system can determine the optimum route to a particular network. If the value is higher, that is preferred.

  15. How do you interpret communities in the context of BGP?

    Ans: A BGP community is a transitive BGP attribute that BGP peers can recognise and communicate. BGP routes sent between two BGP peers are given a BGP community.

    Two 16-bit sections of a 32-bit integer make constitute a community. The AS number for the community is the first 16 bits, and the next 16 bits are a unique number assigned by the AS. Each AS number is unique, and each online community is also unique. 

  16. What are some examples of BGP timers?

    Ans: There are primarily two types of BGP timers:
    Keep-alive Interval: This parameter determines the time in seconds between keep-alive messages being sent. The keep-alive timer is set to 60 seconds by default.
    Hold Time: This parameter sets the time span in seconds after which the neighbour is considered unavailable. 180 seconds is the default hold time.

  17. What exactly does the term "keepalive" mean?

    Ans: It's used to keep track of BGP neighbours and identify those who aren't active. In keepalive messages, just the packet header is seen (19 octets in length). When the frequency of sending keepalive messages is set to 0, they are not delivered.

  18. Can BGP (Border Gateway Protocol) be used instead of any IGP (Internal Gateway Protocol)?

    Ans: The interviewer can ask this type of yes-no question also. So let’s find the solution to this question in our BGP interview questions and answers section.

    No, we can't utilise BGP in place of any IGP since BGP connects autonomous systems, whereas IGP operates within autonomous systems.

  19. What are the different types of communities, and why are they used?

    Ans: Four well-known communities can be mentioned by name:
    a.) No-export:  It prohibits the route from broadcast to eBGP peers outside of the local AS.
    b.) No-advertise: This option prohibits the route from being advertised to internal or external peers.
    c.) Internet: The route can be publicised outside of the local AS via the internet.
    d.) Local-AS: It restricts the route from broadcast to eBGP or confederate peers outside of the local AS.

  20. What is the goal of BGP MED?

    Ans: BGP MED's primary goal is to impact how other autonomous systems interact with your autonomous system(AS) to reach a specific prefix. BGP MED is a property only broadcast to nearby ASes, rather than the entire network. The path will be preferred more when the MED decreases.

  21. Is authentication possible with BGP?

    Ans: The interviewer can also ask about the authentication with BGP. So here we are discussing the answer to this question in our BGP interview questions and answers article.

    Yes! Authentication is possible with BGP. The MD5 authentication is supported by BGP.

  22. Is the next-hop affected by iBGP (internal BGP) sessions?

    Ans: In iBGP sessions, the next-hop property obtained from eBGP peers is retained. This is why it's crucial to have an internal route to the next hop. The BGP route is unreachable elsewhere. 

    Use the next-hop-self neighbour command or provide the network that the next hop belongs to in the IGP to force the router to advertise itself as the next hop rather than the external peer.

  23. What is the value of BGP AD?

    Ans: Routers employ the administrative distance (AD) metric to identify the best path.
    It aids the router in selecting the best route when there are two or more distinct routes to the same destination from two different routing protocols.

    The AD value is used to rank routes from most desired to least liked in Preference.
    The most-preferred route's AD value is the lowest, while the least-preferred route's AD value is the highest.

  24. What is synchronisation, and how does it affect BGP routes in the IP routing table?

    Ans: BGP should not broadcast until all routers in your AS have learned about it through IGP. Suppose your AS is responsible for routing traffic from another AS to a third AS. Before advertising a route to external peers, BGP waits for it to propagate within the AS. If a BGP router with synchronisation disabled cannot validate iBGP learned routes in its IGP, it does not add them to the routing table.

    Issue the no synchronisation command under router BGP to disable synchronisation. As a result, BGP cannot validate iBGP routes in IGP.

  25. What is DUAL, exactly?

    Ans: The DUAL abbreviation stands for Diffusing Update Algorithm. EIGRP uses the DUAL to determine the best routes to a destination. It supports classless routing. As a result, EIGRP routing updates will now include subnet mask information. Discontinuous networks and variable-length subnet masks(VLSM) are now possible.

  26. What is the EIGRP protocol, and how does it work?

    Ans: The Enhanced Interior Gateway Routing Protocol, or EIGRP, is a distance vector routing protocol for IP, AppleTalk, and NetWare networks.

  27. What is EGP, and how does it work?

    Ans: EGP is an acronym for "Exterior Gateway Protocol." It's a protocol for sharing data between gateway hosts in autonomous systems close to each other.

  28.  What does route dampening accomplish?

    Ans: Route dampening reduces the effect of downstream autonomous systems' route flaps on local and upstream autonomous systems.

  29. In general, which routes will affect by route dampening?

    Ans: This type of logical question can also be asked in the interview. So, we are discussing the answer to this question in our BGP interview questions and answer section.

    Only EBGP(External BGP)  routes are affected by route dampening.

  30. How much RAM should your router have to get your ISP's entire BGP routing table?

    Ans: The amount of memory needed to store BGP routes is determined by various parameters:
    a.) Including Router.
    b.) The number of alternate paths accessible.
    c.) Route dampening.
    d.) Community.
    e.) The number of all routes defined.
    f.) BGP attributes.
    g.) VPN configurations.

    It's difficult to estimate the amount of RAM needed to keep a specific number of BGP routes without knowing these factors. To store a whole global BGP routing table from one BGP peer, Cisco typically recommends a router with at least 512 MB of RAM.
    However, it's critical to comprehend how to save memory and achieve effective routing without needing to acquire the entire Internet routing table.
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Frequently Asked Questions

What are the drawbacks to BGP?

The failure of BGP to solve some significant security vulnerabilities and several operational ones are two of its primary flaws. BGP's design and widespread use have hindered previous attempts to secure inter-domain routing.

What is the significance of BGP?

BGP ensures network stability by allowing routers to react to route failures: if one way fails, a new path is found promptly. BGP makes routing decisions based on paths defined by network administrators' rules or network policies.

Where is BGP routing most commonly used?

BGP (Border Gateway Protocol) is the Internet's routing protocol. BGP selects the most efficient routes for delivering Internet traffic, similar to how the post office sets the most efficient ways for sending mail.


We have addressed the most frequently asked BGP interview questions and answers.

We hope this article helped you in gaining knowledge about BGP and Networking. To learn more about Computer Networking, you can visit our articles on Computer Networks.

This article is related to networking protocols and network security. So, it is recommended to see our articles on Network Protocols and  Network Security.

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