Top 17 CCNA Interview Questions and Answers

Cisco Certified Network Associate or CCNA is one of the most popular networking certifications that validates fundamental networking skills. With increasing demand for networking professionals, CCNA certified candidates are in high demand. To land a networking job role, candidates need to pursue CCNA courses from the top institutes to prepare and clear the interview round successfully. 

This guide covers the top 17 CCNA interview questions that are commonly asked during networking job interviews. Spend time understanding the concepts behind each question to confidently answer them.

CCNA Interview Questions and Answers

Below are the set of questions and answers that you must learn to enhance your chances of cracking CCNA interviews. Take a look.

1. What is the OSI Model?

The Open Systems Interconnection (OSI) model is a conceptual framework used to describe the functions of a networking system by segmenting it into 7 abstraction layers – Physical, Data Link, Network, Transport, Session, Presentation and Application layer. Each layer uses the layer below it and provides services to the layer above it. This logical grouping of network functions aids protocol design and understanding network communication flow.

2. Differences between TCP and UDP?

TCP and UDP are the two main transport layer protocols. TCP provides connection-oriented reliable data transmission with flow control and error checking mechanisms. UDP is a connectionless protocol that offers best-effort delivery without reliability, integrity or validation. TCP is used for applications like file transfer where reliability is critical, while UDP suits media streaming requiring minimal latency.

Related: When does DNS use TCP or UDP?

3.Explain the DHCP process.

DHCP or Dynamic Host Configuration Protocol is a network protocol that automates TCP/IP configuration on devices connecting to a network. When a host links to the network, it broadcasts a DHCP discover message. The DHCP server sends an offer with an IP address. Host requests the offered IP and gets assigned the lease. It informs the server of the assignment and can now communicate on the network using leased IP details for the lease time.

4. Types of Network Cables

The common physical network cable types are:

• Twisted Pair cables like CAT5, CAT5e, CAT6 used for Ethernet networks supporting a variety of speeds.
• Coaxial cables used for earlier Ethernet and cable modem networking.
• Fiber optic cables used for high bandwidth, secure networking over long distances. Varieties include Single Mode and Multimode fiber.
• Wireless cables transmit network data through radio signals using technologies like WiFi, Bluetooth, NFC, etc.

5.Which protocol is used for VLAN configuration?

802.1Q is an industry-standard networking protocol that allows virtual LAN (VLAN) tagging in Ethernet networks for assigning ports to VLANs through configuration on switches. By grouping devices into logical segments, VLANs improve security, manageability and optimize bandwidth usage. Switches establish VLAN associations using 802.1Q frame tags to identify packets for designated VLANs.

6.Difference between Switching and Routing.

Switching operates at data link layer (Layer 2) of OSI model to move frames between local area network (LAN) segments. It forwards frames to the destination MAC address using the MAC address table. Routing operates at the network layer (Layer 3) using IP addresses. It forwards packets between different networks/IP subnets by considering factors like routing tables, protocols, metrics and routes frames through multiple network hops using routers.

Related: MAC Address vs IP Address 

7. Explain the function of Subnet Mask

A subnet mask is a 32-bit number used to divide an IP address into a network address and host address in a TCP/IP network. It allows logical allocation of IP addresses in a network by specifying the number of high-order bits assigned to the network part and low-order bits to the host part. Functions include identification of broadcast address, identification of interface for routing etc.

8.Functions of Router

Key functions of a router include:

• Interconnecting multiple networks to enable communication between them.
• Forwarding packets using routing protocols to establish optimal routes.
• Maintaining dynamic routing tables to determine next hop for packets.
• Securing networks by implementing access control lists (ACLs) and firewall policies.
• Load balancing traffic across multiple equal-cost paths.
• Troubleshooting and monitoring network performance using protocols like ICMP.

9.Explain the use of wildcard mask.

Wildcard mask or inverse mask is used to indicate all host bits are relevant for matching purposes as opposed to subnet mask which indicates network/host division. It is created by inverting all host bits of the subnet mask to binary 1s. It simplifies creation of ACL rules by specifying TCP/UDP port ranges using wildcards. For example, 0.0.0.255 wildcard mask allows any source IP while 0.0.0.0 only matches certain IP blocks.

10.Explain Spanning Tree Protocol

Spanning Tree Protocol (STP) prevents bridge loops in switched LAN topologies. If multiple active paths exist between switches due to incorrect cabling or port configurations, it can cause broadcast radiation leading to network failure. STP designates one switch port as a root port and blocks all others on a looped LAN segment, creating a tree topology. PortFast and UplinkFast are STP variants improving convergence.

11.Explain the function of ARP

Address Resolution Protocol (ARP) resolves IP addresses to MAC addresses within local area networks. When a node wants to communicate with another node, it broadcasts an ARP request mentioning the IP address. The node with matching IP replies informs its MAC, updating the ARP cache. Subsequent packets for that destination IP are sent directly using the learned MAC address. This dynamic mapping aids Ethernet communication at the data link layer.

12.Differences between HSRP and GLBP

Hot Standby Router Protocol (HSRP) and Gateway Load Balancing Protocol (GLBP) are alternatives for providing transparent failover of first-hop gateway routers. HSRP establishes an Active-Standby router model assigning a virtual IP and MAC address. GLBP provides a similar gateway redundancy with Active-Standby roles but also balances traffic across multiple devices. GLBP enhances scalability while HSRP is simpler to configure.

13.Explain NAT and its types

Network Address Translation (NAT) allows mapping of private IP addresses used within a local network to public IP addresses used on the Internet. Key NAT types are:

• Static NAT maps one private IP to a public IP on a 1:1 basis.
• Dynamic NAT maps many private IPs to fewer public IPs dynamically on demand.
• Port Address Translation (PAT/NAT Overload) maps multiple private IPs and ports to a single public IP by multiplexing at port level. It conserves public IPs.

Related: NAT vs PAT – IP Address Translation Explained

14.Explain the use of VLAN

Virtual LANs logically segment the physical Ethernet network into different broadcast domains without changing the existing group connections. Devices are associated with VLANs using port configuration or 802.1q tagging. Benefits include increased security, improved performance through broadcast control, flexible network changes, and device virtualization for security zones. Routers are needed for inter-VLAN communication.

15.Explain Classful and Classless IP addressing

Original IP addresses were divided into 5 classes (A, B, C, D, E) based on network/host bit allocations limiting available subnets. However, Classless Inter-Domain Routing (CIDR) was introduced for more flexible subnetting not based on classes. It uses variable length subnet masks (VLSM) to efficiently allocate IP addresses based on actual network size requirements rather than class rules.

16.EIGRP Metrics and Components

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector routing protocol by Cisco. It uses a composite metric based on bandwidth, delay, reliability and load to determine the best path. Key components are – Neighbour Discovery using Hellos, Reliable Transport using sequence numbers, DUAL algorithm for topology updates, Scalability through auto-summarization.

17.What is NAT and PAT translation? Explain with examples.

Network Address Translation (NAT) allows mapping of private IP addresses used within a local network to public IP addresses used on the Internet.
Static NAT maps a private IP to a public IP in a 1:1 ratio.
NAT Overload or Port Address Translation (PAT) maps multiple private IP addresses and ports to a single public IP address using port multiplexing.
For example, a router with public IP 202.54.10.20 can map:

Private IP: 192.168.1.10 port 5000 to Public IP: 202.54.10.20 port 5000
Private IP: 192.168.1.11 port 6000 to Public IP: 202.54.10.20 port 6000

It conserves limited public IP addresses.

Final Thoughts

These are some of the commonly asked interview questions for CCNA level networking professionals. Understanding these concepts thoroughly will help candidates confidently clear interviews and launch their networking career. Regular practice and hands-on experience are necessary to keep concepts up-to-date as technologies evolve constantly in this field. Good luck!