954 questions from 114 interviews · updated from reports 2013-2025
About
Cisco is a technology company that makes networking hardware, software, security products, and collaboration tools used by businesses and internet providers. In India, it commonly hires for software engineer, software engineering intern, network engineer, systems engineer, and related technical roles.
The roles that come up most are Software Engineer, Software Engineering Intern and Software Engineer Intern. This covers 114 candidate interviews reported from 2013 to 2025. Most sat it at entry level (61 of 111 that recorded a level), with 42 internship interviews alongside. Among the 83 that recorded either route, arrivals split between campus drives (61, 73%) and off-campus applications (22, 27%). Most questions fall under CS fundamentals and DSA.
Ans. A switch connects devices within the same local network, while a router connects different networks, such as a home LAN to the internet. A switch forwards frames using MAC addresses, usually at layer 2. A router forwards packets using IP addresses, usually at layer 3, and chooses paths between networks.
Ans. An operating system manages hardware resources and provides services for programs, such as process scheduling, memory management, file systems, device access and security. The key idea is abstraction: it hides hardware details behind interfaces, while coordinating safe and efficient sharing of CPU, memory, storage and input or output devices.
Ans. Use Floyd’s slow and fast pointer method to detect the loop, then remove it by finding the node where the cycle starts and setting the previous node’s next pointer to null. After slow and fast meet, move one pointer to head and advance both one step at a time. Time is O(n), space is O(1).
Ans. TCP is connection-oriented and reliable, while UDP is connectionless and faster but unreliable. TCP guarantees ordered delivery, retransmits lost packets, and handles flow and congestion control. UDP sends datagrams without setup, ordering, or delivery guarantees, making it useful for real-time traffic like streaming, gaming, DNS, and VoIP.
Ans. Reverse it by walking through the list once and redirecting each node’s next pointer to the previous node. Keep three pointers: previous, current, and next, so you do not lose the remaining list. At the end, previous becomes the new head. Time is O(n), space is O(1).
Ans. The OSI model has seven layers: physical, data link, network, transport, session, presentation and application. They describe how data moves from raw bits on a medium, through framing, routing and reliable delivery, up to user-facing protocols. The key idea is separation of concerns, so each layer provides services to the one above.
Ans. Use two pointers: move slow one node at a time and fast two nodes at a time, then slow is at the middle when fast reaches the end. This needs no extra data structure, runs in O(n) time, and uses O(1) space. For an even length list, this usually returns the second middle node.
Ans. A binary tree is a general tree where each node has at most two children, while a trie is a prefix tree used mainly to store strings by characters. The key difference is that a trie's path represents a key or prefix, making prefix lookup efficient, usually in time proportional to word length.
Ans. Computer networking is how devices communicate by following agreed protocols for addressing, routing, transmission and error handling. The key idea is layering: models like TCP/IP separate concerns, so applications use transport protocols such as TCP or UDP, which rely on IP for routing packets across networks.
Ans. Memory management in C is mostly manual: the programmer decides when to allocate and free heap memory. Local variables usually live on the stack, global and static variables have static storage, and dynamic memory comes from the heap using malloc, calloc, realloc, and free. The key risk is freeing incorrectly or forgetting to free, causing bugs or leaks.
Ans. Computer networks connect devices so they can exchange data using agreed protocols. The key idea is layering: each layer handles one responsibility, such as physical transmission, routing, reliable delivery or application behaviour. Important concepts include IP addressing, TCP versus UDP, DNS, routing, latency, bandwidth, congestion, packet loss and security.
Ans. Use expand around centres: for each index, expand once for an odd-length palindrome and once between indices for an even-length palindrome, tracking the best start and length. The key detail is handling both centre types. This uses only a few variables, runs in O(n squared) time, and uses O(1) extra space.
Ans. Use a sliding window and a hash map of each character’s most recent index to find the longest substring without repeats. Move the right pointer through the string; if a character was seen inside the current window, move the left pointer just after its previous index. Track the maximum window length. Time is O(n), space is O(k).
Ans. Use a sliding window with two pointers and a map from character to its last seen index. Move the right pointer through the string, and when a repeated character appears inside the current window, move the left pointer just after its previous position. Track the maximum window length. This runs in O(n) time.
Ans. Design it as a read-heavy social media platform with separate services for identity, media upload, feed generation, follows, likes, comments, notifications and search. Store metadata in a relational or wide-column store, media in object storage behind a CDN, and use queues for async processing. The hardest part is scalable feed ranking and fan-out.
Ans. The TCP/IP model is a layered networking model that describes how data is sent across networks and the internet. It has four layers: application, transport, internet, and network access. The key idea is encapsulation: each layer adds its own information, then the receiving side removes it in reverse order.
Ans. Use Floyd’s cycle detection with two pointers, slow and fast, starting at the head. Move slow one node at a time and fast two nodes at a time. If they ever meet, there is a loop. If fast reaches null, there is no loop. This runs in O(n) time and O(1) space.
Ans. Deadlock is a state where two or more processes are permanently blocked because each is waiting for a resource held by another process. The key point is that none can continue without external intervention. It typically requires mutual exclusion, hold and wait, no preemption, and circular wait to occur.
Ans. Thrashing is a state where an operating system spends most of its time swapping pages between memory and disk instead of executing processes. It usually happens when there is not enough physical memory for the active working sets, causing constant page faults, very low CPU utilisation, and poor overall performance.
Ans. Use Brian Kernighan’s method: repeatedly replace n with n & (n - 1) and increment a counter until n becomes zero. Each operation removes the lowest set bit. The answer is the counter value. This uses no extra data structure, runs in O(number of set bits), and uses O(1) space.
Ans. TCP is connection-oriented and reliable, while UDP is connectionless and faster but does not guarantee delivery. TCP orders packets, retransmits lost data, and provides flow and congestion control. UDP sends datagrams with minimal overhead, so it is useful for real-time traffic like video calls, gaming, DNS, or streaming where some loss is acceptable.
Ans. Use two pointers, one at the start of the string and one at the end, and compare characters while moving inward. If any pair differs, it is not a palindrome; if the pointers meet or cross, it is. This uses no extra data structure and runs in O(n) time with O(1) space.
Ans. TCP is connection-oriented and reliable, while UDP is connectionless and does not guarantee delivery, order, or duplicate protection. TCP uses handshakes, acknowledgements, retransmission, flow control, and congestion control, so it is slower but safer. UDP has lower overhead and latency, so it suits streaming, gaming, VoIP, DNS, and cases where speed matters more than perfect delivery.
Ans. An IP address identifies a device’s location on a network, while a MAC address identifies the network interface hardware. IP addresses are logical and can change when a device moves networks. MAC addresses are usually fixed and are used for local network delivery, while IP is used for routing between networks.
Ans. Use a Node containing data and a next pointer, and keep a head pointer for the list. Implement insert at head by relinking the new node to head, delete by finding the previous node, search by traversal, and display by walking from head. Insert at head is O(1); search, delete, and traversal are O(n).
Ans. Use breadth first search with a queue. Put the root in the queue, then repeatedly remove the front node, visit it, and add its left and right children if they exist. This visits nodes level by level from left to right. The time complexity is O(n), and the space complexity is O(w), where w is the maximum width.
Ans. A process is an independent running program with its own memory space, while a thread is a smaller unit of execution within a process that shares that process’s memory. Processes are more isolated and cost more to create or switch between. Threads are lighter, but shared memory makes synchronisation and race conditions important.
Ans. Start from the top-right element and eliminate one row or one column at a time. If the current value equals the target, return found. If it is greater, move left. If it is smaller, move down. This works because rows and columns are sorted. Time complexity is O(m + n), space is O(1).
Ans. The SDLC phases are planning, requirements analysis, design, implementation, testing, deployment and maintenance. Planning defines scope and feasibility, requirements capture what users need, design describes the architecture, implementation builds it, testing verifies quality, deployment releases it, and maintenance fixes issues and improves the software over time.
Ans. Object Oriented Programming is a way to design software around objects that combine data and behaviour. The main concepts are encapsulation, abstraction, inheritance and polymorphism. The most important detail is that good OOP models responsibilities clearly, hides internal state, and lets code change through interfaces rather than tightly coupled implementation details.
Ans. Computer networks let devices exchange data using layered protocols, mainly the TCP/IP model. The key idea is separation of responsibilities: IP handles addressing and routing, TCP provides reliable ordered delivery, UDP provides faster best-effort delivery, DNS maps names to IP addresses, and application protocols such as HTTP define how services communicate.
Ans. TCP/IP works by splitting data into packets, addressing and routing them with IP, and using TCP to deliver them reliably and in order between applications. IP is best effort and may lose or reorder packets, while TCP adds sequence numbers, acknowledgements, retransmissions, flow control and congestion control.
Ans. Light one candle at both ends and the other at one end. The first candle burns out in 30 minutes, because both ends consume its full hour of wax. At that moment, light the second end of the other candle. It has 30 minutes of burn time left, so burning from both ends takes 15 minutes. Total: 45 minutes.
Ans. Split the string into words, then output the words in reverse order joined by a single space. The key detail is to ignore leading, trailing, and repeated spaces while collecting words. Use an array or list to store words. This takes O(n) time and O(n) extra space.
Ans. ARP maps an IP address to a MAC address on a local network, RARP maps a MAC address to an IP address, and DHCP automatically assigns IP configuration to hosts. ARP is still widely used for local delivery, RARP is largely obsolete, and DHCP also provides details like gateway, subnet mask, and DNS servers.
Ans. Turn on switch 1 for several minutes, then turn it off. Turn on switch 2 and leave switch 3 off. Enter the bulb room. The lit bulb is controlled by switch 2. The unlit but warm bulb is controlled by switch 1. The unlit and cold bulb is controlled by switch 3.
Ans. To convert a queue into a stack, dequeue each element from the queue and push it onto an auxiliary stack. This reverses the order, so the last element removed from the queue becomes the first popped from the stack. The time complexity is O(n) and extra space is O(n).
Ans. An IPv4 header contains fields for version, header length, service type, total length, identification, fragmentation flags and offset, TTL, protocol, header checksum, source address, destination address, and optional options. The most important point is that routers mainly use destination address and TTL, while fragmentation fields support splitting packets across smaller links.
Ans. DHCP automatically gives a device network settings, while DNS translates human-readable domain names into IP addresses. DHCP typically uses Discover, Offer, Request and Acknowledge to assign an IP address, subnet mask, gateway and DNS server. DNS is hierarchical and cached, querying resolvers, root, TLD and authoritative servers when needed.
Ans. Structure padding in C is the unused space a compiler inserts between structure members, or at the end, to satisfy alignment requirements of the target machine. This makes member access faster or valid for the hardware, but it increases sizeof the structure. Member order can change padding, so binary layouts should not be assumed casually.
Ans. IP addresses are assigned from managed address ranges, not simply generated at random. Global blocks are allocated by IANA to regional registries, then to ISPs or organisations. A device usually receives an address automatically through DHCP, or it is configured manually. IPv4 uses 32 bits, while IPv6 uses 128 bits.
Ans. The OSI model is a seven-layer framework for understanding network communication: Physical, Data Link, Network, Transport, Session, Presentation and Application. Data is encapsulated as it moves down the layers and decapsulated on receipt. Each layer has a clear role, from sending bits to routing packets, reliable delivery, formatting data and supporting user-facing protocols.
Ans. Implement a Trie using nodes that store a map from character to child node and a boolean marking the end of a word. To insert or search, walk character by character, creating nodes for insert and failing early for search if a child is missing. Insert, search, and prefix lookup take O(L) time, where L is the string length.
Ans. CPU scheduling algorithms decide which ready process gets the CPU next. Common algorithms include First Come First Served, Shortest Job First, Round Robin, Priority Scheduling and Multilevel Queue. The key trade-off is between throughput, response time, waiting time and fairness, with pre-emptive algorithms allowing the OS to interrupt a running process.
Ans. A URL identifies a resource and tells a client how to access it. It has parts such as scheme, host, port, path, query string and fragment. The browser resolves the host with DNS, opens a connection, often TLS, sends an HTTP request, receives a response, then renders or downloads it.
Ans. The TCP/IP protocol suite is the set of networking protocols that lets devices communicate over the internet and most modern networks. IP handles addressing and routing packets between machines, while TCP provides reliable, ordered delivery using connections, acknowledgements, retransmission and flow control. Common application protocols like HTTP, DNS and SMTP run on top.
Ans. Pick a real conflict where you helped move the team towards a decision, not one where you blame others. Emphasise listening first, separating facts from opinions, clarifying shared goals, and agreeing next steps. Interviewers listen for maturity, calm communication, accountability, respect for different views, and evidence that the outcome improved.
Ans. MapReduce is a disk-based batch processing model, while Apache Spark is a faster general-purpose engine that keeps intermediate data in memory. The key difference is performance and flexibility: Spark supports batch, streaming, SQL and machine learning workloads, whereas MapReduce mainly suits large, reliable, sequential batch jobs.
Ans. REST is an architectural style that usually uses HTTP methods and lightweight formats like JSON, while SOAP is a strict protocol using XML messages and formal contracts via WSDL. REST is simpler and common for public APIs; SOAP is heavier but offers built-in standards for security, transactions and reliability in enterprise systems.
Ans. A network switch connects devices on a local network by forwarding Ethernet frames only to the port where the destination device is connected. It learns which MAC addresses are reachable on each port by reading source addresses in incoming frames. If the destination is unknown, it floods the frame to other ports.
Ans. A router sends packets by reading the destination IP address, looking it up in its routing table, and forwarding the packet to the best next hop. The key detail is longest prefix match: the most specific matching route is chosen. It also decrements TTL, updates checksums, and sends the frame out the selected interface.
Ans. IP fragmentation is the process of splitting a large IP packet into smaller fragments so they can pass over a network link with a smaller maximum transmission unit. Each fragment carries identification and offset information so the destination can reassemble the original packet. If any fragment is lost, the whole packet cannot be rebuilt.
Ans. Routing is the process of choosing paths for data packets to travel from a source network to a destination network. Routers use routing tables to decide the next hop for each packet. These tables are built manually or by routing protocols, using metrics such as hop count, cost, delay, or link reliability.
Ans. State your assumptions, then work through the puzzle step by step. Look for patterns, constraints, edge cases, or invariants, and test a simple example before generalising. If there is a numerical answer, explain how you reached it. If information is missing, say what you would need and avoid guessing blindly.
Ans. Build it as a client heavy graph viewer with a backend API serving nodes, edges and metadata, a layout engine producing positions, and a renderer using Canvas or WebGL for large graphs. The key detail is scalability: cluster, filter and progressively load subgraphs so interaction stays fast instead of rendering the whole graph at once.
Ans. Pick a real, low-drama work conflict where you stayed professional and improved the outcome. Emphasise listening first, checking facts, separating the person from the problem, agreeing next steps, and following up. Interviewers listen for maturity, accountability, calm communication, and evidence that you can protect working relationships while still addressing issues directly.
Ans. Use rates. If a pipe fills a tank in x hours, its rate is 1/x tank per hour. If a pipe empties it in y hours, its rate is -1/y. Add all active rates to get the net rate. Time taken equals 1 divided by the net rate, adjusting for any given starting level.
Ans. Model the network as a graph, with devices as nodes and links as edges. Identify the topology, then check paths, degrees, redundancy, bottlenecks, and failures. For shortest route questions, trace all valid paths systematically. For reliability questions, remove the stated node or link and see whether the graph stays connected.
Ans. Choose a real situation with tension, pressure, or disagreement, but not one that makes you look careless. Emphasise how you stayed calm, understood the issue, communicated clearly, involved the right people, and took ownership. Interviewers listen for judgement, resilience, teamwork, and a positive result or lesson learned.
Ans. Use relative speed to reduce two moving objects to one. If they move towards each other, add their speeds. If they move in the same direction, subtract the slower speed from the faster speed. Then apply distance equals speed multiplied by time. Keep units consistent, usually converting km/h to m/s when needed.
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Candidate interviews most often cover CS fundamentals (58%) and DSA (27%).
Candidate interviews show an average of 4.1 rounds per experience, with a typical sequence of Online test → Technical → Managerial → HR. Individual interview paths can vary.
Among questions with a recorded difficulty, the mix is easy 43%, medium 53%, hard 3%.