101 CCNA Engineer Interview Questions with Expert Answers [2025]

Networking Fundamentals

1. What is the OSI model, and what are its seven layers?

The OSI model standardizes network functions into seven layers:

• Physical: Transmits raw bits (e.g., cables, hubs).
• Data Link: Ensures error-free frame transfer using MAC addresses (e.g., switches).
• Network: Handles IP addressing and routing (e.g., routers).
• Transport: Manages reliable data transfer (e.g., TCP, UDP).
• Session: Controls sessions between applications.
• Presentation: Translates data, managing encryption/compression.
• Application: Supports user applications (e.g., HTTP, DNS).
  It simplifies network design and troubleshooting.

2. How does the TCP/IP model differ from the OSI model?

The TCP/IP model has four layers:

• Network Access: Combines OSI Physical/Data Link (e.g., Ethernet).
• Internet: Matches OSI Network (e.g., IP).
• Transport: Aligns with OSI Transport (e.g., TCP).
• Application: Merges OSI Session/Presentation/Application (e.g., HTTP).
  TCP/IP is practical and internet-focused, unlike the theoretical OSI model.

3. What is an IP address, and how do IPv4 and IPv6 differ?

An IP address identifies devices on a network.

• IPv4: 32-bit, decimal (e.g., 192.168.1.1), ~4.3 billion addresses.
• IPv6: 128-bit, hexadecimal (e.g., 2001:0db8::1), vast address space, supports auto-configuration.
  IPv6 is vital in 2025 for IoT and scalability.

4. What is subnetting, and why is it used?

Subnetting divides a network into smaller subnets using a subnet mask (e.g., 255.255.255.0). It optimizes IP usage, reduces broadcast traffic, and enhances security by segmenting networks (e.g., VLANs for departments). Example: 192.168.1.0/24 split into 192.168.1.0/25 yields 126 hosts per subnet.

5. What is the difference between a hub and a switch?

• Hub: Layer 1, broadcasts to all ports, inefficient.
• Switch: Layer 2, forwards frames using MAC addresses, supports VLANs, efficient.
  Switches are standard for modern LANs.

6. What is a MAC address, and its role?

A 48-bit hexadecimal identifier (e.g., 00:1A:2B:3C:4D:5E) in a NIC, used at the Data Link layer for local frame delivery.

7. What is ARP, and how does it work?

Address Resolution Protocol (ARP) maps IP addresses to MAC addresses in a LAN. A device broadcasts an ARP request, and the target responds with its MAC, stored in the ARP table.

8. What is DNS, and why is it critical?

Domain Name System (DNS) translates domain names to IP addresses, enabling user-friendly network navigation.

9. What is DHCP, and how does it simplify management?

Dynamic Host Configuration Protocol (DHCP) auto-assigns IP addresses, subnet masks, gateways, and DNS servers, reducing manual configuration and IP conflicts.

10. What are VLANs, and their benefits?

Virtual LANs segment devices on the same physical network, improving:

• Security: Isolates traffic (e.g., VLAN 20 for HR).
• Performance: Reduces broadcasts.
• Management: Organizes networks (e.g., vlan 20, name HR).

Routing Essentials

11. What is a router, and how does it operate?

A Layer 3 device that forwards packets between networks using IP addresses and routing tables, leveraging protocols like OSPF or RIP.

12. What is the difference between static and dynamic routing?

• Static: Manually configured (e.g., ip route 192.168.2.0 255.255.255.0 192.168.1.2), for small networks.
• Dynamic: Auto-updates via protocols (e.g., OSPF), for large networks.

13. What is OSPF, and how does it function?

Open Shortest Path First (OSPF) is a link-state protocol using Dijkstra’s algorithm for shortest-path routing, ideal for scalable enterprise networks.

14. What is RIP, and what are its limitations?

Routing Information Protocol (RIP) uses hop count, limited to 15 hops, slow convergence, and less scalability than OSPF.

15. What is EIGRP, and why is it unique?

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco hybrid protocol with fast convergence, low bandwidth use, and IPv4/IPv6 support via DUAL.

16. What is a routing table, and its contents?

Lists paths to networks, including destination, next hop, metric, and protocol source (e.g., show ip route).

17. What is a default route?

Directs traffic to a gateway when no specific route exists (e.g., ip route 0.0.0.0 0.0.0.0 192.168.1.1).

18. What is administrative distance?

A value (0–255) ranking routing protocol reliability (e.g., static = 1, OSPF = 110). Lower is preferred.

19. What is the show ip route command used for?

Displays routing table details for troubleshooting.

20. What is route summarization?

Combines routes (e.g., 192.168.1.0/24, 192.168.2.0/24 to 192.168.0.0/22) to reduce table size.

Switching Technologies

21. What is a switch, and how does it work?

A Layer 2 device forwarding frames via MAC address tables, enabling efficient, collision-free LANs.

22. What is the difference between a switch and a bridge?

• Switch: Hardware-based, multi-port, VLAN support, faster.
• Bridge: Software-based, fewer ports, slower.

23. What is a collision domain, and how does a switch affect it?

A segment where collisions occur. Switches create one collision domain per port, eliminating collisions.

24. What is a broadcast domain, and how does a VLAN affect it?

Where broadcast packets reach all devices. VLANs segment broadcast domains for efficiency.

25. How do you configure a VLAN on a Cisco switch?

Steps:

1. configure terminal.
2. vlan 10.
3. name SALES.
4. interface fa0/1, switchport mode access, switchport access vlan 10.

26. What is a trunk port?

Carries multiple VLANs using tagging (e.g., 802.1Q). Configured with switchport mode trunk.

27. What is Spanning Tree Protocol (STP)?

Prevents switching loops by blocking redundant paths, using a root bridge.

28. What is a root bridge in STP?

Elected by lowest Bridge ID (priority + MAC), it’s the reference for STP path calculations.

29. What is EtherChannel?

Bundles physical links for increased bandwidth/redundancy (e.g., channel-group 1 mode active).

30. What is the show mac address-table command used for?

Shows MAC-to-port mappings for connectivity troubleshooting.

IP Addressing and Subnetting

31. What is a subnet mask?

Divides an IP into network/host portions (e.g., 255.255.255.0 or /24) for routing.

32. What are private IP address ranges?

• 10.0.0.0–10.255.255.255 (/8).
• 172.16.0.0–172.31.255.255 (/12).
• 192.168.0.0–192.168.255.255 (/16).
  Non-routable on public internet.

33. How do you calculate hosts in a subnet?

Formula: 2^(32 - subnet mask bits) - 2. Example: /26 yields 62 usable hosts.

34. What is a network address?

First subnet address (e.g., 192.168.1.0/24), identifies the subnet.

35. What is a broadcast address?

Last subnet address (e.g., 192.168.1.255/24), sends packets to all subnet devices.

36. What is NAT, and how does it work?

Network Address Translation maps private to public IPs. Types:

• Static: One-to-one.
• Dynamic: IP pool.
• PAT: Multiple private IPs to one public IP via ports.

37. What is APIPA?

Automatic Private IP Addressing (169.254.0.1–169.254.255.254) assigns IPs when DHCP fails.

38. What is the show ip arp command used for?

Displays IP-to-MAC mappings for troubleshooting.

39. How do you configure NAT on a Cisco router?

Steps:

1. interface fa0/0, ip nat inside; interface fa0/1, ip nat outside.
2. ip nat inside source list 1 interface fa0/1 overload.
3. access-list 1 permit 192.168.1.0 0.0.0.255.

40. What is a loopback address?

A virtual address (e.g., 127.0.0.1) for testing, always up unless the device fails.

Transport Layer Protocols

41. What is the difference between TCP and UDP?

• TCP: Reliable, connection-oriented (e.g., HTTP).
• UDP: Faster, connectionless (e.g., DNS).

42. What is a TCP three-way handshake?

1. Client sends SYN.
2. Server responds SYN-ACK.
3. Client sends ACK.
   Ensures reliable connection setup.

43. What are port numbers?

Identify services (0–65535, e.g., HTTP: 80) for multiplexing over one IP.

44. What is flow control in TCP?

Regulates data rate via sliding windows to avoid receiver overload.

45. What is the purpose of TCP sequence numbers?

Tracks segment order for reassembly and loss detection.

46. What is the difference between well-known and ephemeral ports?

• Well-Known: 0–1023 (e.g., HTTPS: 443).
• Ephemeral: 1024–65535, for temporary client connections.

47. What is UDP’s primary advantage?

Low overhead, ideal for real-time applications (e.g., VoIP).

48. What is the show tcp command used for?

Shows TCP connection details for debugging.

Network Security

49. What is an Access Control List (ACL)?

Filters traffic by IP/port/protocol.

• Standard: Source IP (e.g., access-list 10 permit 192.168.1.0 0.0.0.255).
• Extended: Source/destination, port, protocol.

50. How do you secure a Cisco router?

• Strong passwords: enable secret .
• SSH: crypto key generate rsa, line vty 0 4, transport input ssh.
• Disable Telnet.
• Use ACLs.
• Enable logging.

51. What is a firewall, and how does it work?

Controls traffic via rules at Layer 3/4, blocking unauthorized access.

52. What is a VPN, and how does it secure data?

Creates encrypted tunnels (e.g., IPsec, SSL) for confidentiality and integrity.

53. What is port security?

Restricts switch port access to specific MACs (e.g., switchport port-security maximum 2).

54. What is 802.1X?

Authenticates devices via RADIUS/TACACS+ for secure network access.

55. What is the difference between WPA2 and WPA3?

• WPA2: AES, vulnerable to brute-force.
• WPA3: Stronger SAE encryption, 2025 Wi-Fi standard.

56. What is symmetric vs. asymmetric encryption?

• Symmetric: One key (e.g., AES), fast.
• Asymmetric: Public/private keys (e.g., RSA), secure but slower.

Wireless Networking

57. What is an SSID?

Wi-Fi network name, broadcasted to identify networks (e.g., Guest, Corporate).

58. What is the difference between 2.4 GHz and 5 GHz Wi-Fi?

• 2.4 GHz: Longer range, more interference (802.11b/g/n).
• 5 GHz: Faster, less interference, shorter range (802.11a/n/ac/ax).

59. What is a wireless access point (WAP)?

Connects wireless devices to wired networks, configured with SSIDs/security.

60. What is a wireless controller?

Centrally manages WAPs for configuration, security, and roaming.

61. What is QoS in wireless networking?

Prioritizes traffic (e.g., VoIP) for low latency in congested Wi-Fi.

62. What are beacons in Wi-Fi?

Periodic signals from WAPs announcing SSID and capabilities.

63. What is the difference between ad-hoc and infrastructure Wi-Fi?

• Ad-Hoc: Direct device connections without WAP.
• Infrastructure: Uses WAP for enterprise/home networks.

IP Services

64. What is SNMP?

Simple Network Management Protocol monitors devices via agents/managers (e.g., bandwidth).

65. What is NTP, and why is it important?

Network Time Protocol synchronizes clocks for accurate logs/security (e.g., ntp server 0.pool.ntp.org).

66. What is syslog?

Collects device logs for centralized troubleshooting (e.g., logging host 192.168.1.100).

67. What is HSRP?

Hot Standby Router Protocol provides router redundancy (e.g., standby 1 ip 192.168.1.1).

68. What is VRRP?

Virtual Router Redundancy Protocol, an open-standard HSRP alternative with simpler setup.

69. What is ICMP?

Internet Control Message Protocol handles diagnostics (e.g., ping, traceroute).

70. What is the show running-config command?

Displays current device configuration for auditing.

71. What is the difference between ping and traceroute?

• Ping: Tests reachability/round-trip time.
• Traceroute: Maps packet paths and latency.

Troubleshooting Techniques

72. How do you troubleshoot connectivity issues?

1. Check cables/LEDs.
2. Verify IPs: show ip interface brief.
3. Test reachability: ping .
4. Trace path: traceroute .
5. Check logs: show logging.
6. Review ACLs/firewalls.

73. What does show ip interface brief do?

Shows interface status/IPs for configuration checks.

74. What is a CRC error?

Indicates corrupted frames due to faulty cables or duplex mismatches.

75. What is the debug command?

Monitors processes in real-time (e.g., debug ip packet), but use cautiously due to CPU load.

76. What is packet loss, and how do you diagnose it?

Packets failing to reach destinations; diagnose with ping, traceroute, or packet analyzers.

77. What does high latency indicate?

Congestion, long routes, or device issues, impacting real-time apps.

78. What is the show version command?

Shows IOS version, uptime, and hardware for compatibility checks.

79. How do you identify a switching loop?

Symptoms: Broadcast storms, high CPU. Check with show spanning-tree.

Network Automation and SDN

80. What is Software-Defined Networking (SDN)?

Separates control/data planes for centralized, programmable network management.

81. What is the role of APIs in automation?

Enable programmatic configuration/monitoring (e.g., REST, NETCONF).

82. What is NETCONF?

Uses XML messages for device configuration, supporting automation.

83. What is Cisco DNA Center?

Centralized platform for network automation, analytics, and assurance.

84. What is intent-based networking?

Translates business goals into automated network policies.

85. What is Ansible for network automation?

Uses YAML playbooks to automate Cisco device configurations.

86. Why is programmability important in 2025?

Reduces errors, speeds deployments, and supports IoT/5G scalability.

Advanced Routing and Switching

87. What is BGP?

Border Gateway Protocol exchanges routes between autonomous systems, used in internet/enterprise networks.

88. What is a designated router (DR) in OSPF?

Elected by priority/router ID, manages link-state updates to reduce traffic.

89. What is LACP?

Link Aggregation Control Protocol negotiates EtherChannel for redundancy/bandwidth.

90. What is a VRF?

Virtual Routing and Forwarding creates separate routing tables for multi-tenant/VPN setups.

91. What is MSTP?

Multiple Spanning Tree Protocol maps VLANs to single STP instances for efficiency.

Practical Cisco Configurations

92. How do you configure a static route?

ip route 192.168.2.0 255.255.255.0 192.168.1.2.

93. How do you configure OSPF?

1. router ospf 1.
2. network 192.168.1.0 0.0.0.255 area 0.
3. router-id 1.1.1.1.

94. How do you configure a trunk port?

interface fa0/1, switchport mode trunk, switchport trunk allowed vlan 10,20.

95. How do you configure port security?

interface fa0/1, switchport mode access, switchport port-security, switchport port-security maximum 2.

96. How do you configure SSH?

1. hostname R1.
2. ip domain-name example.com.
3. crypto key generate rsa.
4. line vty 0 4, transport input ssh.

97. How do you configure a basic ACL?

access-list 10 permit 192.168.1.0 0.0.0.255, interface fa0/0, ip access-group 10 in.

Emerging Trends

98. Why is IPv6 critical in 2025?

Supports IoT/5G with vast addresses, no NAT, and built-in IPsec.

99. What is zero-trust architecture?

Assumes no trust, requiring continuous authentication for security.

100. What is 5G’s impact on networking?

Enables high-speed, low-latency apps like autonomous vehicles with advanced QoS.

101. What are top CCNA skills for 2025?

• Routing/switching (OSPF, VLANs).
• IPv6 configuration.
• Automation (Python, Ansible).
• Security (ACLs, zero-trust).
• SDN familiarity.

Tips for Acing Your CCNA Interview

• Practice with Packet Tracer/GNS3.
• Master commands like show ip route.
• Stay updated on IPv6, SDN, and zero-trust.
• Explain concepts clearly.
• Study Cisco’s official guides and practice labs.