Arista Networks Software Engineer Interview: Questions & Prep (2026)

Arista Networks builds cloud networking switches and routers, and its Extensible Operating System (EOS) is written primarily in C++. The Software Engineer interview process at Arista typically runs across multiple rounds: a recruiter or HR screen, one or two technical phone or video rounds, and a full onsite or virtual panel with 3-5 interviews covering coding, networking, and systems design. Candidates report that the bar is high on both data structures and networking theory, reflecting the nature of EOS development.

As of July 2026, knok's job radar shows 6 open Software Engineer roles at Arista Networks, out of 5,395 Software Engineer openings across India. Arista's Indian engineering presence is primarily in Bangalore, which accounts for the largest share of Software Engineer demand in the country. The roles are typically backend and systems-focused, with a strong preference for candidates who have networking or operating systems experience.

Candidates at Arista Networks Software Engineer interviews commonly report questions across three areas: networking theory, data structures and algorithms, and systems design.

1. Explain how BGP works and why large networks prefer it over interior gateway protocols.
2. How does OSPF calculate the shortest path, and what data structure does it use internally?
3. What is VXLAN, and how does it solve the scale limitations of traditional VLANs in data centers?
4. Write a function to detect a cycle in a linked list. What is the time and space complexity?
5. Implement an LRU cache with O(1) get and put operations.
6. How would you design a system that processes millions of routing table updates per second without dropping updates?
7. Explain the difference between a process and a thread. How does context switching work at the kernel level?
8. Write code to serialize and deserialize a binary tree.
9. How do you troubleshoot a scenario where two hosts can communicate via ping but a specific application is failing?
10. What is the difference between control plane and data plane in a network device? How does Arista separate them in EOS?
11. Describe a situation where you improved the performance of a critical piece of code. What did you measure and what did you change?
12. How does spanning tree protocol prevent broadcast storms, and what are its main drawbacks?

Q: Describe a situation where you debugged a complex production issue.

*Situation:* At my previous company, a routing daemon began behaving erratically under heavy BGP update load, causing intermittent packet drops for customers during network convergence events.

*Task:* As the on-call engineer, I needed to identify the root cause and restore stability within our incident SLA window.

*Action:* I started by correlating log timestamps with the drop events and noticed the drops coincided with memory usage spikes in the routing process. I confirmed that the BGP session itself was staying up and that the BGP control-plane port was not being blocked by any firewall rule. Using heap profiling tools, I traced the spike to a route-map parser that was allocating temporary objects on every received UPDATE message but not freeing them promptly. I wrote a fix that reused a pre-allocated buffer pool and tested it in a staging environment with a BGP traffic replay.

*Result:* After the patch, memory usage under the same load stayed flat and the packet drops stopped. I also added a monitoring alert on heap growth rate so the same pattern would page the team before causing customer impact in future.

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Q: How would you design a system to handle millions of routing table updates per second?

*Situation:* My previous team needed a high-throughput route reflector to handle BGP UPDATE floods during large-scale network convergence, where the existing single-threaded processor was becoming a bottleneck.

*Task:* I was asked to redesign the update-processing pipeline to meet the new throughput target without sacrificing correctness.

*Action:* I proposed splitting the pipeline into three stages: ingestion, parsing, and RIB update. I used lock-free queues between stages so that slow parsing could not block ingestion. For the RIB itself I chose a compressed trie structure to support fast longest-prefix-match lookups, and I sharded the table by address family to allow parallel writes. I also added batching so that multiple updates to the same prefix within a short window would be coalesced before hitting the RIB lock.

*Result:* Load tests showed the redesigned pipeline handled the target throughput with average update latency well within acceptable bounds. The design was adopted and later extended to support additional address families.

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Q: Implement an LRU cache with O(1) get and put.

*Situation:* This was a live coding question in a technical round. The interviewer asked for a clean implementation with correct time complexity, then probed for thread-safety considerations.

*Task:* Design and implement the cache from scratch, explain the choice of data structures, and discuss how the solution would change in a concurrent environment.

*Action:* I used a hash map for O(1) key-to-node lookup and a doubly linked list to maintain access order, with the most recently used item at the head. On every get or put I moved the accessed node to the front. When capacity was exceeded I removed the tail node and its map entry simultaneously. For the thread-safety follow-up I explained that a single mutex would serialize all operations, that a read-write lock would help for read-heavy workloads, and that sharding the cache by key hash would reduce contention further.

*Result:* The solution passed all test cases. The interviewer noted the thread-safety discussion was exactly the direction they wanted to explore and asked a follow-up about deadlock prevention.

For networking theory questions, start by naming the OSI layer the protocol operates at, then explain the problem the protocol solves, and finally walk through how it solves it step by step. Arista interviewers care about depth, not just recall, so be ready to go one level deeper on any point you raise.

For coding questions, think aloud before you type. State the brute-force approach first, then explain why you are optimizing. Write clean, compilable C++ where possible, since candidates report Arista strongly prefers C++ over Python for systems-level roles.

For systems design questions, use a three-step structure: clarify the scale and constraints, sketch a high-level architecture, then drill into the component most likely to be the bottleneck. For Arista specifically, be ready to separate control-plane concerns from data-plane concerns in any design that touches networking.

For behavioral questions, use the STAR structure (Situation, Task, Action, Result). Keep the Situation brief (one or two sentences), spend most of your time on Action, the choices you made and why, and always end with a measurable or observable Result. Avoid vague endings like 'it went well.'

Arista interviewers, based on candidate reports, look for three things above everything else.

Genuine networking depth. It is not enough to know that BGP is a path-vector protocol. Interviewers will probe why it is path-vector, what problems that solves compared to link-state, and how EOS implements BGP policy. Study OSPF, BGP, spanning tree, VXLAN, and ECMP at the level where you can draw packet flows on a whiteboard.

Strong C++ fundamentals. EOS is a large C++ codebase. Expect questions on memory management, RAII, move semantics, smart pointers, and STL containers. Candidates who can explain why they chose a particular container and what its cache behavior is tend to get positive feedback.

Systems thinking. Arista's products run at line rate on large-scale networks. Interviewers want to see that you think about latency, throughput, memory footprint, and failure modes automatically, not only when prompted. In any design question, volunteer a discussion of what happens when a component fails.

A four-week plan that candidates have found effective for Arista SE roles:

In parallel, keep a short note of every concept you feel shaky on after each session and revisit it the next morning. Spaced repetition matters more than cramming.

While you are deep in preparation, knok checks 150+ job sites nightly, applies to Software Engineer roles that match your resume, and messages HR for you, so you keep finding new openings without the extra effort.

Jumping into code without clarifying. Candidates report that Arista interviewers specifically watch whether you ask clarifying questions before coding. State your assumptions out loud before you write a single line.

Treating networking questions as rote recall. Saying 'BGP is used for inter-AS routing' without explaining why it is better suited than OSPF for that use case will not impress. Go one level deeper on every concept you mention.

Writing Python when C++ is expected. For systems-level roles, defaulting to Python can signal that you are not comfortable with the language Arista uses in production. Ask the interviewer which language they prefer and default to C++ unless told otherwise.

Ignoring failure modes in design questions. A design that only works when everything is healthy is incomplete. Always address what happens when a link goes down, a process crashes, or a message is dropped.

Weak STAR answers with no concrete result. Behavioral answers that end with 'the team was happy' or 'it worked out' leave interviewers with nothing to evaluate. Tie your result to something observable: a metric that improved, an incident that did not recur, or a product that shipped.