Network engineer interview questions

1. Describe OSPF in your own words.
2. SPF areas, the purpose of having each of them
3. Types of OSPF LSA, the purpose of each LSA type
4. What exact LSA type you can see in different areas
5. How OSPF establishes neighboor relation, what the stages are
6. If OSPF router is stucked in each stage what the problem is and how to troubleshoot it
7. OSPF hierarchy in the single or multi areas. Cool OSPF behavior in broadcast and nonbroadcast
8. Draw the diagram of typical OSPF network and explain generally how it works, DR, BDR, election, ASBR, ABR, route redistribution and summarization

STP - Spanning Tree Protocol

1. How it works and the purpose
2. Diff types (SSTP, MSTP, RSTP) Cisco - PVST/PVST+
3. root election
4. Diff. port stages and timing for convergence
5. Draw the typical diagram and explain how diff types of STP work
6. What ports are blocking or forwarding
7. How it works if there are topology changes

1. What are they
2. Diff types
3. Write an example if you want to allow and to deny…
4. Well-known port numbers (DNS - 53 and etc…)

1. What is QOS in computer networking?

Quality of Service (QoS) in computer networks is a set of technologies and mechanisms that manage and prioritize network traffic to ensure reliable performance for critical applications, such as voice and video calls. It achieves this by controlling bandwidth allocation, minimizing latency and jitter, and ensuring applications like VoIP, video streaming, and online gaming function smoothly, even during periods of high network congestion. QoS works by classifying traffic and giving priority to more important data over less time-sensitive data.

2. What is the difference between L2 and L3 Quality of Service (QoS)?
   Answer: The main difference between Layer 2 and Layer 3 QoS is where they operate in the network stack and the scope of their influence.
   • Layer 2 QoS works at the data link layer, typically within a single LAN or between directly connected switches. It uses the 802.1p priority field (3 bits in the VLAN tag) to mark frames with priority levels 0-7. This is sometimes called CoS (Class of Service). L2 QoS decisions are made based on MAC addresses and VLAN information, and the prioritization only has meaning within that Layer 2 domain - once traffic hits a router, those L2 markings don't cross over directly.
   • Layer 3 QoS operates at the network layer and works across routed networks. It uses the DSCP (Differentiated Services Code Point) field in the IP header - 6 bits that allow for 64 different traffic classes. This gives much finer granularity than L2's 8 classes. Because L3 QoS markings are in the IP header, they survive routing and can be honored end-to-end across multiple network segments and administrative domains.
   
   In practice, they often work together. At the network edge, you might classify traffic and mark it at Layer 3 with DSCP values. Switches can then map those DSCP values to Layer 2 CoS values for local switching decisions. When traffic reaches the next router, it reads the DSCP markings again and continues applying QoS policies. Think of L2 QoS as local prioritization within your switched environment, while L3 QoS provides end-to-end traffic management across the broader routed network. Most enterprise networks use both in tandem for comprehensive QoS.
3. How it works

Switching:

1. VLANs: The purpose of VLANs is to segment a single physical network into multiple logical networks for improved security, performance, and management. They allow administrators to group users and devices by department or function, rather than by physical location, which isolates traffic, reduces broadcast domains, and simplifies administration.
2. STP: In networking, STP stands for Spanning Tree Protocol, a Layer 2 protocol that prevents network loops by logically blocking redundant paths while maintaining a loop-free topology. It ensures that there is only one active path between any two devices on a Local Area Network (LAN). STP is critical for creating fault-tolerant networks that are also loop-free, preventing issues like broadcast storms that can bring down the network.
3. How does a Layer 2 switch work with broadcast?

   A Layer 2 switch handles broadcasts by flooding them out all ports except the one it came in on.
   This is fundamentally different from how it handles unicast traffic. Here's what happens:
   

   • Broadcast frames have a destination MAC address of FF:FF:FF:FF:FF:FF. When a switch receives a frame with this destination, it doesn't consult its MAC address table at all - there's no need to look up where "everyone" is. Instead, it simply forwards that frame to every active port in the same VLAN, except the port where it received the frame.
   • The broadcast domain: This flooding behavior means that all devices connected to a switch (or multiple switches connected together) form a single broadcast domain. Every device receives every broadcast. Common examples include ARP requests ("Who has IP address 192.168.1.5?"), DHCP discovery messages, and various network announcements.
   • Why this matters: Broadcasts are necessary for many protocols to work, but they consume bandwidth and require every device to process the frame at least briefly. In a small network, this is fine. But in a large flat network with hundreds or thousands of devices, broadcast traffic can become problematic - this is sometimes called a "broadcast storm" in extreme cases.
   • VLANs provide isolation: This is one reason VLANs exist. Each VLAN is its own broadcast domain, so broadcasts in VLAN 10 won't flood to devices in VLAN 20, even if they're on the same physical switch. A router is needed to move traffic between broadcast domains.
   So essentially, switches are indiscriminate with broadcasts - they just blast them everywhere within that Layer 2 boundary.
   (unicast, multicast, known/unknown traffic)
4. VRRP, GLBP
5. port monitoring and mirroring
6. L3 switch, how it works
7. PIM sparse and dense modes