Top BGP Interview Questions and Answers

Most Asked BGP Interview Question and Answers

1. What are some characteristics of the Border Gateway Protocol?

Here are a few characteristics of the Border Gateway Protocol:

• The primary function of the BGP is to provide communication between two autonomous systems. This is known as the configuration of inter-autonomous systems.
• The BGP also supports the Next-Hop paradigm.
• There remains strong coordination among various BGP speakers within the autonomous system.
• The BGP advertisements offer path information in addition to reachable and subsequent destination pairs. 
• Numerous attributes are used in the routing decision algorithm of BGP.
• The internal neighbors which work inside the same autonomous system use the iBGP.
• There are various policies implemented by the Border Gateway Protocol that are configured by the administrator. 
• The TCP is used in conjecture with the Border Gateway Protocol. 

2. What is the port number used by the Border Gateway Protocol?

The Border Gateway Protocol uses the Transmission Control protocol post number 179.

3. Can routers on different subnets become BGP neighbors?

The BGP is set up frequently between two routers that are connected directly and belong to distinct autonomous systems. These routers hardly require the support of their neighbors on the same subnet. They instead employ a TCP network between the routers to send and receive BGP messages thus allowing the neighboring routers to be on the same or distinct subnets.  

4. In the context of networking what do you mean by Routing Information Protocol? 

The Routing Information Protocol is a dynamic routing protocol that aims to find a path between the source and the destination networks by using the hop count as a routing metric. RIP uses the port 520. The hop count is the number of routers between the source and the destination network. The best route to a network is judged by noticing the path with the fewest hops. It thus enters into the routing table. The maximum number of hops that are allowed by the RIP is 15. A Hop count of 16 is not reachable by the network. 

5. Differentiate between external Border Gateway Protocol and Internal Border Gateway Protocol?

• Internal Border Gateway Protocol (iBGP): Inside autonomous systems, IBGP is used. Its purpose is to feed data to your internal routers. For prefix learning, all devices in the same autonomous system must form a full mesh topology or use either Route reflectors or Confederation.
• External Border Gateway Protocol (eBGP): It is employed between autonomous systems. It's used and deployed at the edge or border router, which connects two or more autonomous systems together. It is the protocol that allows networks from various organizations or the Internet to communicate with one another.

6. In the context of BGP, what does poison reverse mean?

The Poison reverse algorithm is a widely used distance vector algorithm that solves the count to infinity problems. Poison reverse is a routing information protocol technique. When the path information becomes invalid the routers do not remove them instantly from the database. They instead broadcast a hop count of 16 which is an unreachable metric value. This might aid in the elimination of the loop but increases the size of the routing table. It has the ability to break any loop between the neighboring routers right away. 

7.  What do you understand about communities in the context of BGP?

A BGP community is a transitive, optional BGP property that is recognised and sent between BGP peers. A BGP community is a tag that is appended to the BGP routes that are exchanged between two BGP peers. A community is a 32-bit number that is divided into two 16-bit sections. The first 16 bits indicate the community's AS number, while the following 16 bits represent a unique number assigned by the AS. Because each AS number is distinct, each community on the internet is similarly distinct.

8. What do you mean by BGP Message Types? Explain the different Message Types in BGP in detail.

The BGP Message types are used to create a neighbor relationship and exchange parameters such as the autonomous system number and authentication values. A BGP message is made up of two parts: a BGP header and data. The header format is the same in all of the BGP messages. TCP is used to transmit BGP messages (port 179). The length of the message ranges from 19 to 4096 octets. Each BGP message's header is 19 octets long and consists of three fields.

• Open: A BGP adjacency is established using the OPEN message. Before a BGP peering is established, both parties negotiate session capabilities. The BGP version number, ASN of the originating router, Hold Time, BGP Identifier, and other optional characteristics that define the session capabilities are all included in the OPEN message.
• Keepalive: To ensure that the neighbors are still alive, BGP does not rely on the TCP connection status. For every one-third of the Hold Timer agreed upon between the two BGP routers, keepalive messages are exchanged. Generally, the default Hold Time for devices is 180 seconds, so the default Keepalive interval is 60 seconds. No Keepalive messages are delivered between BGP neighbors if the Hold Time is set to zero.
• Update: The Update message can either advertise or remove previously advertised routes, or it can do both. When advertising prefixes, the Update message includes the Network Layer Reachability Information (NLRI), which includes the prefix and related BGP PAs. Only the prefix is included in NLRIs that have been withdrawn. To save needless bandwidth, a UPDATE message can be used as a Keepalive.
• Notification: When a BGP session error is discovered, such as a hold timer expiring, neighbor capabilities changing, or a BGP session reset is requested, a Notification message is sent. The BGP connection is closed as a result of this.

9. Explain the Path selection criteria used in BGP.

BGP seeks to reduce the number of paths available to only one best path; it does not load balance by default. To do so, it looks at the following path properties of any loop-free, synchronized (if synchronization is enabled) routes with an accessible next-hop:

• Pick the route that has the most weight.
• Choose the route with the highest local preference if weight is not specified.
• Choose routes that began with this router.
• Choose the Autonomous System path that is the shortest.
• Select the path with the lowest origin code.
• If the same Autonomous System advertises the available routes, choose the path with the lowest MED.
• Opt for an EBGP route rather than an IBGP route.
• Choose the route that passes through the IGP neighbor with the lowest IGP metric.
• Pick the oldest route.
• Choose the path that passes through the neighbor with the smallest router ID.
• Select the path that passes through the neighbor with the smallest IP address.

FAQs 

1. What Algorithm is used by BGP?

The BGP is set up frequently between two routers that are connected directly and belong to distinct autonomous systems. These routers hardly require the support of their neighbors on the same subnet. They instead employ a TCP network between the reuters to send and receive BGP messages thus allowing the neighboring routers to be on the same or distinct subnets.  

2. Is BGP TCP or UDP?

BGP uses the TCP port 179 to communicate with routers. TCP allows handling of fragmentation, reliability and sequencing of communication pockets.

3. What OSI layer is BGP?

BGP operates on the OSI transport layer 4.

4. How does BGP work?

BGP takes charge of analyzing all the possible paths to a data that has been sent over the internet and selects the best one through which it could be sent. In short, BGP is the protocol that allows data to be routed across the internet. 

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Conclusion

In this article, we have covered the most frequently asked BGP interview questions. If you are preparing for a BGP interview, you can expect questions from networking as well. If you are looking to enhance your skills further, then we would highly recommend you to check Simplilearn’s Post Graduate Program in Full Stack Web Development in collaboration with Caltech CTME. This course will help you enhance your software development skills and make you job-ready.

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