BGP Interview Questions and Answers: The Ultimate Guide for 2023 - IQCode

What is BGP?

BGP, which stands for Border Gateway Protocol, is known as the internet's postal service. Just like how the postal service processes and finds the most efficient way to deliver a letter to its desired destination, BGP analyzes all possible paths for data to take in order to select the best route. This usually involves the data hopping between autonomous systems. BGP facilitates data routing across the internet, allowing users from one end to communicate seamlessly with servers on the other side of the world.

The BGP protocol can link any autonomous system's internet together using any topology, as long as each autonomous system has at least one BGP-capable router connected to the BGP router of another autonomous system. BGP's primary objective is to communicate network reachability information with other BGP systems, which is then used to create an autonomous system graph.

BGP Interview Questions for Freshers

  1. What are the characteristics of the Border Gateway Protocol (BGP)?

Border Gateway Protocol (BGP) Port Number

Which port number does the Border Gateway Protocol (BGP) operate on?

The BGP protocol uses port number 179. 

Can Routers on Different Subnets Become BGP Neighbors?

In BGP, routers can become neighbors and exchange routing information with each other. It is possible for routers on different subnets to become BGP neighbors as long as there is IP connectivity between them. However, it is important to ensure that the routers are properly configured with the correct BGP neighbor statements and that any necessary firewall rules are in place to allow BGP traffic to pass through.

Can you run two BGP processes on the same router?

Yes, it is possible to run multiple BGP (Border Gateway Protocol) processes on the same router. In fact, it can be useful in certain scenarios.

However, it is important to note that running multiple BGP processes can consume additional resources on the router, so it's important to evaluate the impact on performance and make sure the router has enough resources to handle the additional load.

Understanding Routing Information Protocol (RIP) in Networking

In networking, Routing Information Protocol (RIP) is a dynamic routing protocol that uses distance-vector algorithm to determine the best path for data transmission between networks. It is designed to handle small to medium sized networks and is used to share information about the network topology between routers. RIP uses hop count as a metric for calculating the best path, where a hop refers to the number of routers that a packet must pass through to reach its destination. RIP also includes mechanisms to prevent routing loops and the counting to infinity problem. RIP has been largely replaced by more efficient protocols such as Open Shortest Path First (OSPF) in larger networks, but is still widely used in smaller networks.

Difference between Internal Border Gateway Protocol (iBGP) and External Border Gateway Protocol (eBGP)

The main difference between iBGP and eBGP lies in their scope and the way they exchange routing information.

  • iBGP is used for routing within an autonomous system (AS) and is used to exchange routing information between routers that belong to the same AS.
  • eBGP is used for routing between autonomous systems and is used to exchange routing information between routers that belong to different ASes.

Additionally, iBGP maintains the next-hop information while eBGP does not. This means that iBGP peers must have a full mesh connectivity while eBGP peers do not.

Another difference lies in the way they treat the autonomous system path (AS_PATH) attribute. When an eBGP router receives a route from another AS, it will add its own AS number to the AS_PATH attribute. In contrast, iBGP does not modify the AS_PATH attribute when propagating routes within the same AS.

In summary, iBGP is used for internal routing within an autonomous system, while eBGP is used for routing between different autonomous systems.

Understanding Split Horizon in BGP

In the context of Border Gateway Protocol (BGP), Split Horizon is a method used to avoid loops in the network topology. It prevents BGP from advertising a route back to the same Autonomous System (AS) that it learned the route from.

For example, imagine AS1 and AS2 are directly connected and have two routes to a destination network, one via AS3 and another via AS4. Without Split Horizon, AS1 may advertise the route to AS2 through both paths (AS3 and AS4), and AS2 will also advertise both paths to AS1. This can create a loop and cause unnecessary network traffic.

To avoid this problem, Split Horizon is implemented, and AS1 will only advertise the route to AS2 through one path (let's say AS3), and will hold back the second path (AS4) while advertising to all the other AS in the network. This ensures that a loop is prevented, and network traffic is optimized.

Understanding Poison Reverse in BGP

In the context of BGP, Poison Reverse is a technique used to prevent routing loops. When a router receives a route from a neighbor, it sends an update back to that neighbor with the same route, but with an infinite metric value. This tells the neighbor that the route is no longer valid, and the neighbor will stop forwarding traffic to that destination. This mechanism helps to quickly remove invalid routes from the network and prevent packet loss.

Understanding BGP Peers and the Purpose of BGP Peer Groups

In the context of BGP, peers refer to other BGP routers that exchange routing information. These peers can be within the same autonomous system (iBGP peers) or in different autonomous systems (eBGP peers). The primary purpose of BGP peers is to enable the sharing of routing information and enable the best routing decisions for traffic.

BGP peer groups, on the other hand, are a feature of BGP that allows for the efficient configuration of BGP peers. Instead of manually configuring individual BGP neighbors, a BGP peer group allows for the configuration of a group of BGP peers with similar properties, such as the same inbound or outbound policies. This saves time and effort when managing BGP configurations, especially in large-scale networks with many BGP peers.

Is Authentication Possible with BGP? If Yes, Explain How.

BGP (Border Gateway Protocol) supports authentication, which helps in preventing unauthorized access and ensures secure communication between BGP peers. BGP Authentication is a mechanism that allows BGP routers to verify their identity and the identity of their neighbors using a password-based authentication method.

When BGP peers establish a TCP session, they exchange BGP messages containing authentication information. This information includes an MD5 hash that is generated using a shared secret key known to both the routers.

To enable BGP authentication, you need to configure the authentication key on the router. The authentication key must be the same on both the routers exchanging BGP messages. Once authentication is enabled, the routers include the MD5 hash in all BGP messages exchanged between them.

Authentication is necessary when you are running BGP with routers that are not under your direct control or are over a public network. By enabling BGP authentication, you can ensure that unauthorized routers do not participate in the BGP session and prevent attacks such as route hijacking, packet sniffing, and other security threats.

Order of Preference in BGP

In BGP, the order of preference determines which path will be selected when there are multiple options for reaching the same destination. The order of preference is as follows:

1. Highest local preference value 2. Shortest AS path 3. Lowest origin type (IGP Types of Timers in BGP In BGP, there are four types of timers mentioned below: 1. ConnectRetry timer - This timer is used to manage the time to reconnect if a TCP connection is lost. 2. Hold Timer - This timer manages the maximum time allowed for the BGP speaker to remain silent before declaring the opposite end to be not active. 3. KeepAlive Timer - This timer sets the time interval between two consecutive messages to keep the session active. 4. Start Timer - This timer is used to delay the BGP speaker before announcing the BGP session to ensure that all the necessary setup procedures have been completed.

Using BGP instead of IGP?

It is possible to use BGP (Border Gateway Protocol) as an alternative to IGP (Interior Gateway Protocol). However, choosing one over the other depends on the specific networking requirements and goals.

BGP is often preferred for larger networks with multiple connections to different service providers. It is designed for inter-domain routing and can handle large-scale routing more efficiently than IGP. On the other hand, IGP is better suited for smaller networks where the routing is contained within the network.

It is important to consider factors such as network size, complexity, and connectivity needs before deciding whether to utilize BGP or IGP.

Understanding the Different BGP Neighbor Adjacency States


BGP (Border Gateway Protocol) is the standard protocol used by routers to exchange routing information across the internet. When two BGP routers establish a connection, they go through a series of steps to establish a BGP neighbor adjacency. These steps are known as BGP neighbor adjacency states.

The various BGP neighbor adjacency states include:

  • Idle: This is the initial state when the BGP process is not running or when the router is waiting for a TCP connection to be established.
  • Connect: In this state, the BGP router is waiting for a TCP connection to be established with its neighbor.
  • Active: The router is attempting to initiate a TCP connection with its BGP neighbor in this state.
  • OpenSent: During this state, the router has sent an Open message to its neighbor but is waiting for a response.
  • OpenConfirm: In this stage, the router has received a response from its neighbor and is waiting for a configuration message to complete the BGP neighbor adjacency.
  • Established: This is the final stage where both BGP routers have exchanged all necessary information and established a BGP neighbor adjacency.

Understanding the BGP neighbor adjacency states is crucial in troubleshooting connectivity issues between BGP routers. By examining the current state, network engineers can determine where the problem is and take corrective action accordingly.

Types of Attributes in BGP

In BGP, there are several types of attributes that can be used for route selection and manipulation. These attributes include:

1. Well-known mandatory attributes - These are required attributes that must be present in all BGP updates. Examples of well-known mandatory attributes include the AS path and next hop attributes.

2. Well-known discretionary attributes - These are optional attributes that may be included in BGP updates. Examples of well-known discretionary attributes include the local preference and aggregation attributes.

3. Optional transitive attributes - These are optional attributes that may be included in BGP updates, and they are passed on to other BGP routers. Examples of optional transitive attributes include the community and extended community attributes.

4. Optional non-transitive attributes - These are optional attributes that may be included in BGP updates, but they are not passed on to other BGP routers. Examples of optional non-transitive attributes include the originator ID and cluster list attributes.

Understanding these attributes is important for effective BGP route selection and manipulation.

Well-Known BGP Metric Attributes

Some of the well-known BGP metric attributes are:

  • AS Path Length
  • Next Hop
  • Local Preference
  • Weight
  • Multi-Exit Discriminator (MED)

In BGP routing, these attributes are used to determine the best path for data traffic to take. Each attribute plays an important role in the overall routing decision.

Understanding Route Reflectors in the context of Border Gateway Protocol

In the context of Border Gateway Protocol (BGP), a route reflector is a network device that helps to distribute routing information among multiple BGP routers. It is necessary because without a route reflector, BGP routers in a large network would have to be fully meshed, meaning that each BGP router would have to be directly connected to every other BGP router in the network. This could become unmanageable and resource-intensive as the number of routers in the network grows.

Instead, a route reflector allows BGP routers to be organized in a hierarchical manner, with some routers acting as route reflectors and others as clients. The route reflectors collect and then distribute routing information to their clients, thus reducing the number of BGP peer connections needed and making the network more scalable and easier to manage.

BGP Interview Questions for Experienced

18. Explain the difference between Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP).

OSPF is an interior gateway protocol (IGP) used for routing within an autonomous system (AS). It uses link-state routing (LSR) and shortest path first (SPF) algorithm. OSPF calculates the shortest path based on the lowest cost metric, which makes it very efficient for routing within a network.

On the other hand, BGP is an exterior gateway protocol (EGP) used for routing between different autonomous systems. BGP uses path-vector routing, which not only considers the shortest path but also takes into account other factors such as network policies and traffic load. BGP is more scalable than OSPF and can handle a much larger network.

In summary, OSPF is used for internal routing within a network, while BGP is used for communicating between different networks.

// Example of BGP configuration

router bgp 65001

neighbor remote-as 65002

neighbor ebgp-multihop 2

neighbor update-source Loopback0

address-family ipv4

neighbor activate

neighbor prefix-list BLOCK-ROUTES-TO-CUSTOMER in

neighbor route-map SEND-ONLY-BEST out


Differentiating between Hard Reset and Soft Reset in BGP

In the context of Border Gateway Protocol (BGP), a hard reset involves completely terminating the BGP session and resetting all related parameters. This results in the removal of all routes learned from the peer, requiring a full re-advertisement of routes and new route selection.

On the other hand, a soft reset involves resetting the BGP session and related attributes without terminating the connection itself. This results in a refresh of only the affected routes learned from the peer, without the need for a full re-advertisement or new route selection.

In summary, a hard reset in BGP involves a complete termination and reset, while a soft reset only refreshes specific attributes without terminating the session.

Understanding BGP Path Attributes

BGP (Border Gateway Protocol) uses various path attributes to choose the best path to reach a destination network. Some important BGP path attributes include:

  • AS_PATH: Lists the autonomous system numbers in the path to reach the destination network.
  • NEXT_HOP: Identifies the next hop IP address to reach the destination network.
  • LOCAL_PREF: Used to determine the preferred route out of a local autonomous system.
  • MULTI_EXIT_DISC: Used to influence traffic entering an autonomous system with multiple entry points.
  • ORIGIN: Identifies the origin of the BGP route (IGP, EGP or incomplete).

By understanding these attributes, network engineers can fine-tune BGP routing decisions for optimal network performance.

Community Concept in the Context of BGP

In the context of BGP, communities are groups of networks that share common routing policies. They are used to tag or mark different routes and provide information about the routes to neighboring routers. This information can be used by the neighboring routers to make decisions on how to handle traffic and which path to take. Communities in BGP can be used to control routing updates, filter routes, simplify route redistribution, and identify different types of traffic. Overall, communities play a vital role in improving network efficiency and optimizing routing decisions in BGP.

Types of Communities Used in BGP

In Border Gateway Protocol (BGP), communities are used to group destinations and apply specific routing policies. There are four types of communities used in BGP:

1. Well-Known Communities - These communities are pre-defined and recognized by all BGP implementations. There are several well-known communities, such as NO_EXPORT, NO_ADVERTISE, and INTERNET.

2. Optional Transitive Communities - These communities are defined by network administrators and can be passed between autonomous systems (AS) during route advertisements.

3. Optional Non-Transitive Communities - These communities are defined by network administrators but are not passed between ASs during route advertisements.

4. Bogon Communities - These communities are used to identify routes that should not be propagated and are often used by Internet Service Providers (ISPs) to filter invalid or malicious traffic.

Understanding these community types and effectively implementing them can help network administrators ensure that their BGP routing policies are applied correctly and efficiently.

BGP Message Types

BGP (Border Gateway Protocol) is a protocol that routers use to exchange information like routing information and network prefixes with other routers in order to determine the best path for sending data between networks. BGP makes use of different message types to achieve this task. The various BGP message types are:

1. Open Message: The Open message is the first message that is sent between routers and is used to establish a connection between them. The message includes parameters like the version number of BGP, the Autonomous System Number (ASN) of the router, and a Hold Time value.

2. Update Message: The Update message is used by routers to exchange routing information like network prefixes and their associated attributes. This message is critical to the operation of BGP as it is used to update router's routing tables.

3. Notification Message: A Notification message is sent when there is an error in the BGP protocol or when one of the routers wants to terminate the connection. This message includes a message code and a message subcode that describe the error or condition that caused the message to be sent.

4. Keepalive Message: The Keepalive message is used by routers to maintain the connection that was established by the Open message. The routers send this message to each other after a certain interval of time to ensure that the connection is still active and to detect when it fails.

Understanding these different BGP message types is crucial for network engineers who are responsible for maintaining and troubleshooting BGP connections. It is important to note that BGP is a complex protocol and requires a strong understanding of how it works, in order to configure and manage it effectively.

Explanation of the Path Selection Criteria Used in BGP

The Border Gateway Protocol (BGP) is used to exchange routing information between different autonomous systems (AS). When multiple paths are available for a destination, BGP uses a set of criteria to select the best path. Below are the criteria used in BGP path selection in the order of priority:

  1. Weight: The weight is a Cisco-specific parameter that assigns a higher preference to a certain path.
  2. Local Preference: The local preference is used within the same AS to elevate the preference of a path towards a specific destination.
  3. AS Path Length: Paths with the shortest AS Path are preferred.
  4. Origin Code: A path originating from within the AS is preferred over a path learned from another AS.
  5. Multi-Exit Discriminator (MED): For paths learned from the same AS, the path with the lowest MED is preferred.
  6. External Path Type: If all else is equal, paths learned from an external BGP peer are preferred over paths learned from an internal peer.
  7. IGP Metric: An Interior Gateway Protocol (IGP) metric can be used as a tiebreaker if all other criteria are equal.
  8. Path Selection Timer: If none of the above criteria can differentiate between paths, the default timer-based criterion is used to select the best path.

Path selection criteria are important in BGP because they ensure that the best path is chosen, which enables efficient and reliable communication between autonomous systems.

Difference between Border Gateway Protocol (BGP) and Routing Information Protocol (RIP)

BGP and RIP are both routing protocols used in computer networks, but they differ in several ways.

BGP is an external gateway protocol used for routing between different autonomous systems (AS). It is used by Internet Service Providers to exchange routing information between their networks. BGP supports larger and more complex networks and uses more advanced routing policies than RIP.

RIP, on the other hand, is an interior routing protocol used for routing within a single autonomous system. It uses a simpler algorithm than BGP and is generally used in smaller networks. RIP has a limit on the distance between two nodes in the network, which makes it less suitable for larger networks.

In summary, BGP is used for larger and more complex networks that require advanced routing policies, while RIP is used for smaller networks with simpler routing requirements.

Difference between Enhanced Interior Gateway Routing Protocol (EIGRP) and Border Gateway Protocol (BGP)

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a routing protocol that is used to find the most efficient path for data transmission within a single autonomous system (AS) network. On the other hand, the Border Gateway Protocol (BGP) is used to exchange routing information between different autonomous systems (AS).

While EIGRP is a classless protocol that supports Variable Length Subnet Masks (VLSMs) and uses Diffusing Update Algorithm (DUAL) for loop avoidance, BGP is a classless protocol that supports Network Address Translation (NAT) and uses the path-vector algorithm for loop avoidance.

Overall, EIGRP is suited for use within a single network, while BGP is more appropriate for use between multiple networks or ASs.

// Sample code to implement EIGRP and BGP

// EIGRP configuration

router eigrp 1

// BGP configuration

router bgp 65100
 neighbor remote-as 65200
 neighbor remote-as 65300

TTL Security in BGP

TTL (Time to Live) security is a mechanism used in Border Gateway Protocol (BGP) to prevent attacks on the network by limiting the number of hops a packet can travel. The TTL value of a packet is set by the sender and is decremented by one at every hop. If the TTL value becomes zero, the packet is dropped.

The need for TTL security in BGP is to prevent attackers from launching DDoS (Distributed Denial of Service) attacks by injecting false routes with a low TTL value. This can cause a routing loop, which can lead to network congestion and even a complete outage. By using TTL security, BGP routers can be configured to drop packets that have a low TTL value, which prevents attackers from causing harm to the network.

Types of Loop Prevention Mechanisms in BGP

In BGP, there are several mechanisms to prevent loops:

1. AS Path Attribute:

BGP checks if the received route contains its own AS number in the AS path attribute. If it does, the route is rejected.

2. Next Hop Attribute:

BGP also checks if the next-hop IP address in the received route is reachable. If it is not, the route is rejected.

3. Route Reflection:

Route reflection is a technique used in BGP to avoid routing information loops when using BGP confederations. In this method, the routes received from a client AS are reflected to other clients without modifying the AS path. This way, loops are avoided.

4. Confederations:

BGP Confederations are used to avoid BGP loops within a large autonomous system. The AS is split into smaller sub-ASes, and each sub-AS is configured as a separate autonomous system.

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