NETWORK ENGINEER INTERVIEW PREP

Network Engineer Interview Questions and How to Answer Them

Network engineering interviews mix fundamentals (OSI, routing protocols, VLANs) with troubleshooting scenarios. Interviewers want to hear a calm, layered method and real examples from networks you have run.

Walk me through what happens when you type a URL into a browser.
How does OSPF choose the best path, and what is the role of the DR/BDR?
BGP: how does a router pick the best path among several?
A user says 'the network is slow'. How do you troubleshoot?
Explain VLANs, trunking and how inter-VLAN routing works.
What is the difference between a stateful firewall and an ACL?
How does spanning tree prevent loops, and what goes wrong when it fails?
Tell me about a network outage you handled.

1. Walk me through what happens when you type a URL into a browser.

Why they ask: Tests whether you can connect DNS, ARP, TCP, TLS and HTTP into one coherent flow instead of reciting layers.

How to answer:

DNS resolution (cache, resolver, recursive lookup) to get the IP.
ARP for the default gateway MAC, then routing hop by hop.
TCP three-way handshake, then TLS handshake for HTTPS.
HTTP request/response; mention where a firewall, proxy or load balancer would sit in your environment.

2. How does OSPF choose the best path, and what is the role of the DR/BDR?

Why they ask: Checks link-state fundamentals and whether you understand flooding control on multi-access segments.

How to answer:

OSPF builds an LSDB and runs SPF (Dijkstra); cost is based on interface bandwidth (reference bandwidth / interface bandwidth).
DR/BDR reduce adjacencies on broadcast segments — routers form full adjacency only with DR/BDR.
Election: highest priority, then highest router ID; non-preemptive.
Mention a real tuning example, e.g. adjusting reference bandwidth for 10G links.

3. BGP: how does a router pick the best path among several?

Why they ask: Senior roles expect you to know path selection and how you would influence inbound vs outbound traffic.

How to answer:

Recite the key order: weight, local preference, locally originated, AS-path length, origin, MED, eBGP over iBGP, IGP metric, then tie-breakers.
Outbound traffic: local preference / weight. Inbound: AS-path prepending, MED, communities.
Give one example from a dual-ISP or data-center design you have worked on.

4. A user says 'the network is slow'. How do you troubleshoot?

Why they ask: Scenario questions reveal your method. Interviewers want structure, not guesses.

How to answer:

Scope it: one user, one site, one app, or everyone? Since when? What changed?
Work the layers: physical/interface errors, duplex, utilisation, then L2 (STP changes, MAC flapping), L3 (routing, latency, packet loss), then the application.
Use data: ping/traceroute, interface counters, NetFlow, firewall logs, packet capture.
Close with how you confirmed the fix and prevented recurrence.

5. Explain VLANs, trunking and how inter-VLAN routing works.

Why they ask: Core switching knowledge that every network role relies on.

How to answer:

VLANs segment broadcast domains; access ports carry one VLAN, trunks carry many with 802.1Q tags.
Native VLAN is untagged — mention the security practice of changing it from VLAN 1.
Inter-VLAN routing via router-on-a-stick or SVIs on an L3 switch; most enterprises use SVIs.
Bonus: mention VTP risks or why you prune VLANs on trunks.

6. What is the difference between a stateful firewall and an ACL?

Why they ask: Tests security awareness, especially for roles that touch Palo Alto, Fortinet or Check Point.

How to answer:

ACLs are stateless — each packet is matched independently, so return traffic needs its own rule.
Stateful firewalls track sessions and allow return traffic automatically.
Next-gen firewalls add application identification, user-ID, IPS and TLS inspection.
Give an example of a rule-base cleanup or a policy you designed.

7. How does spanning tree prevent loops, and what goes wrong when it fails?

Why they ask: Checks L2 resilience knowledge and real-world incident experience.

How to answer:

Root bridge election (lowest bridge ID), then root/designated/blocked ports by path cost.
RSTP converges much faster than classic STP; mention PortFast and BPDU Guard on edge ports.
Failure = broadcast storm: CPU spikes, MAC table flapping, outage.
Describe how you would detect and contain it.

8. Tell me about a network outage you handled.

Why they ask: Behavioral + technical: ownership, communication and root-cause skills.

How to answer:

Situation: impact and urgency in one sentence.
Action: how you isolated the cause and who you kept informed.
Result: time to restore, plus the permanent fix.
Lesson: what monitoring, change control or design change followed.

More interview guides

Any job title works in CareerCue — these are just a starting point. See all fields →

Cybersecurity Analyst8 questions →Cloud Engineer8 questions →Software Engineer8 questions →Data Analyst8 questions →IT Support / Help Desk8 questions →DevOps Engineer6 questions →Product Manager6 questions →Data Scientist6 questions →

Mind going blank in the real interview?

CareerCue works for any job in any field. It listens to the question and shows a short cue based on your resume and the role, so you can answer in your own words. Free to try, no credit card.

Try CareerCue free →