Technical interview guide

Routing & Switching Fundamentals

How traffic gets forwarded within a local network (switching) versus across different networks (routing).

Read: 45 min
Practice MCQs: 25
Interview QA: 25
Edition: v6
Editorial status: Reviewed
Relevant for: Network Engineer

Scope: Current IETF router, CIDR, OSPFv2/v3, BGP, eBGP policy, VRRP, BFD, EVPN, VXLAN, Cisco Layer 2/STP, and Juniper BGP references reviewed 2026-09-04.

Interview QA

Treat each question like a live interview question: answer out loud first (structure, assumptions, tradeoffs), then open the model answer to spot gaps and rehearse a tighter follow-up.

Curated: 2026-10-05 · Written: 2026-10-05 · Reviewed: 2026-10-03

QA-1

Diagnose MAC flapping between switch ports.

QA-2

Control excessive unknown-unicast and broadcast traffic.

QA-3

Troubleshoot a VLAN access-port mismatch.

QA-4

Diagnose a VLAN that fails across an otherwise healthy trunk.

QA-5

Design and validate a spanning-tree topology.

QA-6

Troubleshoot an aggregate with one suspended member.

QA-7

Explain route lookup for a destination with default, aggregate, and specific routes.

QA-8

Diagnose a route present in control plane but not forwarding.

QA-9

Validate ECMP health and diagnose one bad path.

QA-10

Design safe route aggregation.

QA-11

Explain the OSPF control-to-forwarding pipeline.

QA-12

Troubleshoot OSPF neighbors stuck below FULL.

QA-13

Design OSPF areas and summarization.

QA-14

Explain BGP route import, selection, and export.

QA-15

Design inbound and outbound traffic engineering with BGP.

QA-16

Build safe BGP import and export policy.

QA-17

Design and test first-hop redundancy.

QA-18

How do default gateways, static routes, and dynamic routing protocols differ in operation, and when should each be used?

QA-19

How does Layer 2 Port Security work on switchports, and what are the operational differences between protect, restrict, and shutdown violation modes?

QA-20

Diagnose asymmetric routing through stateful middleboxes.

QA-21

Compare Static NAT, Dynamic NAT, and Port Address Translation (PAT). How do their translation mappings and state tables operate under load?

QA-22

Validate a path after routing convergence.

QA-23

Design safe redistribution between OSPF and BGP.

QA-24

Design routing and switching observability.

QA-25