Preparing for a Network Engineer interview means you need to be clear on both fundamental and advanced concepts. Below are detailed answers to some commonly asked questions. Each answer is written in a flowing style that you can study, remember, and deliver confidently in interviews.

1. What is a Network?

A network is a collection of two or more devices connected together to share resources, exchange data, or communicate. Networks can be small, like a home Wi-Fi, or very large, like the internet. They are built using devices such as switches, routers, firewalls, and servers, and they allow communication between users, applications, and services. Networks are essential for businesses today because they make resource sharing, collaboration, and internet connectivity possible.

2. What is the size of an IP address?

There are two versions of IP addresses: IPv4 and IPv6. IPv4 uses a 32-bit address, which provides around 4.3 billion unique addresses. It is written in dotted decimal format, such as 192.168.1.1. Because the number of devices in the world has grown far beyond 4 billion, IPv6 was introduced. IPv6 uses a 128-bit address, which supports an almost unlimited number of addresses. It is written in hexadecimal format, such as 2001:db8::1.

3. Explain the Layers of the TCP/IP Model.

The TCP/IP model is a 4-layer model used to describe data communication in real-world networks.

1. Network Access Layer: Handles physical delivery of frames using MAC addresses, switches, and cables.
2. Internet Layer: Provides logical addressing and routing using IP addresses.
3. Transport Layer: Ensures end-to-end communication using TCP for reliable delivery or UDP for faster, connectionless delivery.
4. Application Layer: Provides services and protocols like HTTP, FTP, DNS, and SMTP that users interact with.

Compared to the OSI model’s seven layers, TCP/IP combines them into four practical layers that are directly implemented in networking.

4. Explain EIGRP.

EIGRP, or Enhanced Interior Gateway Routing Protocol, is an advanced distance-vector routing protocol developed by Cisco. It is known as a hybrid protocol because it combines features of both distance-vector and link-state protocols. EIGRP uses metrics like bandwidth, delay, reliability, and load to calculate the best path, but by default it only considers bandwidth and delay. It uses the DUAL algorithm (Diffusing Update Algorithm) to ensure loop-free and efficient routing. EIGRP is classless, supports VLSM, fast convergence, unequal-cost load balancing, and uses protocol number 88. It exchanges routing updates with neighbors using multicast address 224.0.0.10.

5. What is the multicast address of OSPF?

OSPF, or Open Shortest Path First, uses multicast addresses to send updates. The two important multicast addresses are 224.0.0.5, which is used to send updates to all OSPF routers, and 224.0.0.6, which is used to send updates specifically to the Designated Router (DR) and Backup Designated Router (BDR).

6. What is the election criteria of DR and BDR in OSPF?

In OSPF, when routers are connected to a multi-access network like Ethernet, a Designated Router (DR) and Backup Designated Router (BDR) are elected to reduce unnecessary flooding of LSAs. The election is based on the following criteria:

1. The router with the highest priority value is elected as DR.
2. If priorities are the same, the router with the highest Router ID (RID) becomes DR.
3. The second-highest becomes BDR.
4. If priorities are set to 0, the router is not eligible to become DR or BDR.

This process helps reduce overhead and ensures efficient OSPF operation.

7. What is EtherChannel?

EtherChannel is a technology that combines multiple physical links into one logical link between two devices such as switches or between a switch and a router. It increases bandwidth and provides redundancy. If you combine four 1 Gbps links, it behaves like one 4 Gbps link, and if one link fails, traffic is automatically shifted to remaining links. EtherChannel is treated as a single logical interface by protocols like STP, avoiding loops.

8. What are the different modes of LACP?

LACP, or Link Aggregation Control Protocol, is an IEEE standard used to form EtherChannel. The two main modes are:

• Active Mode: The interface actively tries to form an EtherChannel by sending negotiation messages.
• Passive Mode: The interface responds to LACP negotiation but doesn’t initiate.

For an EtherChannel to form with LACP, one side must be Active, or both can be Active. Passive-to-Passive will not form an EtherChannel.

9. What is the difference between eBGP and iBGP?

BGP, or Border Gateway Protocol, is used to exchange routes between autonomous systems. When BGP is running between two routers in the same autonomous system, it is called iBGP (internal BGP). When BGP is running between routers in different autonomous systems, it is called eBGP (external BGP).

• eBGP neighbors are usually directly connected and use different AS numbers.
• iBGP neighbors can be multiple hops away but must be in the same AS.
• eBGP is generally used for communication between organizations or ISPs, while iBGP is used within an organization’s internal network to distribute routes learned from eBGP.

10. What protocol does trunking use?

On switches, trunk ports are used to carry multiple VLANs across a single physical link. To identify which VLAN a frame belongs to, trunking uses IEEE 802.1Q protocol. In this protocol, a VLAN tag is added inside the Ethernet frame to indicate the VLAN ID. This allows a single trunk port to carry traffic from many VLANs at the same time.

11. What are the switch port modes?

Switch ports can be configured in different modes:

• Access Mode: The port belongs to a single VLAN and is used for end devices like PCs.
• Trunk Mode: The port carries multiple VLANs using tagging, usually between switches.
• Dynamic Auto (Cisco proprietary): The port will become trunk if the other side is trunk or desirable.
• Dynamic Desirable (Cisco proprietary): The port actively tries to negotiate trunking with the other side.

12. What is the difference between a Router and a Switch?

A switch operates at Layer 2 and uses MAC addresses to forward frames within a LAN. Each port on a switch is its own collision domain, and switches reduce collisions and improve efficiency. A router, on the other hand, works at Layer 3, uses IP addresses, and connects different networks together. Routers make path decisions using routing tables and can provide NAT, firewall, and WAN connectivity. In short, switches connect devices within a network, while routers connect different networks.

Final Interview Tip

Always answer in a flow: start with a definition, add a detailed explanation, and finish with a real-world example. This makes your answers complete and prevents cross-questions.