Border Gateway Protocol (BGP) is the backbone of the internet that manages how data moves between networks. It was first introduced in 1989 by engineers Yakov Rekhter and Kirk Lougheed as a replacement for the older EGP protocol. Over time, BGP became a core tool for internet service providers, telecoms, and big enterprises because it handles routing decisions at scale. This makes skills in BGP valuable for roles like network engineer and BGP specialist. In this blog, we cover the top 25+ BGP interview questions and answers to help you prepare.
Fun Fact: There are over 70,000 active Autonomous Systems (ASNs) registered worldwide, and every one of them uses BGP to keep the internet connected.
Understanding the BGP Interview Process
BGP Interview Questions for Freshers
Here are the commonly asked BGP interview questions and answers designed to help beginners understand the basics.
1. What is BGP and why is it used in Internet routing?
BGP (Border Gateway Protocol) is the routing protocol that connects different autonomous systems (AS) on the internet. It is used to exchange reachability information and select the best path for data across networks. Without BGP, the internet would not function as a collection of independent networks.
2. What are the key attributes of BGP?
BGP uses attributes to decide the best path. The most important ones are:
● Weight – Cisco-specific, higher is preferred.
● Local Preference – Influences outbound traffic within an AS.
● AS Path – Shorter paths are preferred, also prevents loops.
● Origin – Indicates how a route entered BGP (IGP, EGP, or Incomplete).
● MED (Multi-Exit Discriminator) – Suggests preferred entry point into an AS.
● Next Hop – The IP address to forward traffic.
3. What is the difference between eBGP and iBGP?
| Feature | eBGP (External BGP) | iBGP (Internal BGP) |
|---|---|---|
| Scope | Between routers in different ASes | Within the same AS |
| Usage | At the edge of networks | To share external routes internally |
| AS Path | Adds AS numbers | Does not add AS numbers |
| Next Hop | Usually changes to advertising router | Next hop remains unchanged |
4. What is the role of the AS-PATH attribute in BGP?
The AS-PATH lists the sequence of ASes a route has passed through. It prevents routing loops and is also used in path selection, where shorter paths are preferred.
5. What are the BGP neighbor adjacency states?
BGP has six states:
● Idle
● Connect
● Active
● OpenSent
● OpenConfirm
● Established
A session is considered fully up only in the Established state, where routes are exchanged.
6. Why does BGP use TCP port 179?
BGP uses TCP port 179 to establish reliable sessions between peers. TCP provides error checking, retransmission, and session stability, which are critical for internet routing.
7. Can routers in different subnets become BGP neighbors?
Yes, they can. BGP does not require peers to be in the same subnet. As long as IP reachability exists and TCP port 179 is open, neighbors can form a session even across different subnets.
BGP Interview Questions for Experienced Professionals
Let’s go through BGP interview questions and answers for experienced professionals that focus on advanced concepts.
8. What are the common loop prevention mechanisms in BGP?
The primary method is the AS-PATH attribute. If a router sees its own AS number in the path, it rejects the route. iBGP also uses the split horizon rule, which prevents advertising routes learned from one iBGP peer to another iBGP peer. Route reflectors and confederations scale this further while still preventing loops.
9. How does the Local Preference attribute influence routing?
Local Preference decides which exit point an AS uses when multiple paths exist. A higher value is preferred. This attribute is shared with all routers inside the AS, making it a key tool for traffic engineering.
10. What is the role of the MED (Multi-Exit Discriminator) attribute?
MED is used between different ASes to indicate the preferred entry point into a network. A lower MED value is chosen over higher ones, but it is only compared among routes coming from the same neighboring AS.
11. What is a Route Reflector, and why is it needed?
In large networks, iBGP requires a full mesh of peerings, which does not scale well. A Route Reflector allows one router to redistribute routes to others, reducing the number of peerings required. This simplifies network design without breaking loop prevention.
12. Can BGP support authentication? If yes, how?
Yes, BGP supports MD5 authentication. Both peers configure the same shared password, and all BGP packets are verified using MD5 checksums. This prevents unauthorized peers from establishing sessions and protects against spoofed updates.
Bonus: BGP Interview Questions for CCNA and CCNP Engineers
Here are some bonus questions for experienced CCNA and CCNP engineers to test deeper BGP knowledge and practical skills.
● What is the difference between “weight” and “local preference” in Cisco BGP?
● What is next-hop self and when do you configure it?
● What is an IBGP full mesh requirement and how do route reflectors solve it?
● How does BGP handle route redistribution from OSPF or EIGRP?
● Explain the BGP “hard reset” vs “soft reset” commands on Cisco routers.
BGP Scenario Based Interview Questions
These are BGP protocol interview questions framed around real scenarios to test problem-solving and practical application.
13. You see BGP routes in the table, but they are not installed in the routing table. What could be the reason?
This usually happens when the next hop is unreachable. Another reason could be that a route with a better administrative distance, like OSPF or static, already exists. Policy filters or route maps can also block installation.
14. If two ISPs advertise the same prefix, how does BGP decide the best route?
BGP compares attributes in a defined order. First, it checks Local Preference. Then it looks at AS-PATH length. If they tie, it considers MED, followed by eBGP over iBGP, IGP cost to the next hop, router ID, and finally neighbor IP. The shortest AS-PATH is preferred only if Local Preference is the same.
15. A BGP session is stuck in the Active state. What would you check?
First, I check basic IP connectivity between peers. Then I confirm that TCP port 179 is open and not blocked by firewalls. I also verify correct neighbor IPs and AS numbers in the configuration. Finally, I check for authentication mismatches if MD5 is enabled.
16. Traffic is leaving through the wrong ISP link in a dual-homed setup. How would you fix it?
The first step is to review Local Preference values inside the AS. If outbound traffic is going through the wrong ISP, I increase Local Preference for the desired path. For inbound traffic, I may use AS-PATH prepending or MED to influence the remote AS.
BGP Tricky Interview Questions
Here are BGP tricky interview questions and answers that challenge your understanding and test how well you know the finer details of BGP.
17. Can BGP perform load balancing across multiple paths?
By default, BGP picks only one best path. However, multipath load sharing can be enabled if multiple routes have equal attributes. In Cisco, this is done using the maximum-paths command.
18. What happens if two routes have the same attributes in BGP?
BGP compares tie-breakers in order: eBGP over iBGP, lowest IGP cost to next hop, oldest path, lowest router ID, and finally lowest neighbor IP. The final tie-break ensures only one best path.
19. Why does BGP rely on TCP and not UDP?
BGP requires reliable, connection-oriented delivery. TCP provides sequencing, retransmission, and session persistence. UDP lacks these features, so it is not suitable for inter-domain routing.
20. What is the difference between a hard reset and soft reset in BGP?
A hard reset tears down the session and rebuilds it, which causes traffic disruption. A soft reset re-applies routing policies without dropping the session. It’s the preferred method in production networks.
21. If two routers form iBGP, why is full mesh required?
iBGP does not re-advertise routes learned from another iBGP peer. This prevents loops but forces a full mesh. Route reflectors or confederations are used to scale large networks.
Note: BGP troubleshooting interview questions are often tricky, as they test not just your theoretical knowledge but also how you handle real routing issues.
BGP MCQs
Here are some BGP MCQs to quickly test your knowledge and recall of key concepts.
1. Which BGP message type is used to establish a session?
a) Keepalive
b) Open
c) Update
d) Notification
Answer: b) Open
2. What attribute is used to influence outbound routing within an AS?
a) AS Path
b) Weight
c) Local Preference
d) MED
Answer: c) Local Preference
3. In BGP, which is preferred: shorter AS-PATH or higher local preference?
a) Shorter AS-PATH
b) Higher Local Preference
c) Both are equal in preference
d) Depends on router configuration
Answer: b) Higher Local Preference
4. What does “soft reset” command do in BGP?
a) Restarts the TCP session
b) Clears all routes permanently
c) Reapplies policy and refreshes routes without restarting the session
d) Shuts down the neighbor session temporarily
Answer: c) Reapplies policy and refreshes routes without restarting the session
5. Which state indicates a BGP session is fully established?
a) Idle
b) Connect
c) OpenConfirm
d) Established
Answer: d) Established
6. What is the default administrative distance for eBGP on Cisco?
a) 90
b) 110
c) 20
d) 200
Answer: c) 20
7. Which well-known community value prevents a route from being advertised externally?
a) NO_EXPORT
b) NO_ADVERTISE
c) NO_EXPORT_SUBCONFED
d) INTERNET
Answer: a) NO_EXPORT
How to Prepare for BGP Interview?
Preparing for BGP interviews requires both theory and hands-on practice with real networking scenarios. Here are some tips to help you:
● Study core concepts like AS-PATH, MED, and Local Preference.
● Practice labs on route reflectors, confederations, and troubleshooting cases.
● Go through BGP interview questions and answers CCNP for mid-level roles.
● Review BGP interview questions and answers CCIE for advanced design topics.
● Focus on scenario-based questions and practical troubleshooting, not just definitions.
Wrapping Up
So, these are the 25+ BGP interview questions and answers that will help you practice and build confidence for your next networking interview. Keep learning, keep testing your knowledge, and stay updated with the latest protocols and techniques.
Looking for IT jobs, including BGP roles? Visit Hirist to find the latest tech job openings.
FAQs
BGP utilizes four message types: OPEN, which establishes a session; UPDATE, which shares routing changes; KEEPALIVE, which maintains connectivity; and NOTIFICATION, which terminates sessions when errors occur.
The Multi-Exit Discriminator (MED) is an optional non-transitive attribute used to influence inbound traffic flow. It allows an autonomous system to suggest the preferred entry point to its network when multiple paths exist between two ASes.
Leading technology firms and service providers such as Cisco, Juniper Networks, Amazon, Google, Microsoft, and large Internet Service Providers (ISPs) regularly recruit professionals with strong BGP expertise for network engineering and architecture positions.
CCNP (Cisco Certified Network Professional) is an advanced certification for enterprise networking roles, while CCIE (Cisco Certified Internetwork Expert) is the highest expert-level certification, targeting senior engineers and architects with deep technical proficiency.
