Cisco Network Engineer Interview: Routing, Switching, and Network Security Full Assessment

Background

Let me start with my background. I have a bachelor's in Network Engineering and spent 2 years as a network engineer at a systems integration company, mainly responsible for enterprise network planning, implementation, and operations. Day-to-day, I worked with routers, switches, and firewalls, primarily using Huawei and H3C equipment. Cisco's Network Engineer position was always a goal of mine — Cisco is a giant in the networking equipment field, and working there would mean exposure to larger-scale networks and cutting-edge technologies.

I applied in June through Cisco's career website. About a week later, HR contacted me to schedule the first round. The entire process was three technical rounds plus an HR round, spanning about three weeks. Cisco's interviews place heavy emphasis on professional depth and practical experience — especially understanding network protocols beyond surface-level knowledge. Let me walk through each round in detail.

Interview Process Review

Round 1: Network Fundamentals — OSPF/BGP (about 65 minutes)

My first-round interviewer was a professional-looking network architect. He started with a self-introduction, asked some project background questions, then moved into technical territory.

OSPF deep dive:

The first question carried real weight — "Explain how OSPF works." I covered OSPF area division (backbone area, regular area, Stub area, NSSA), neighbor state machine (Down, Init, 2-Way, ExStart, Exchange, Loading, Full), LSA types (Type 1-7), and SPF algorithm in detail. The interviewer followed up on several points: "What are the DR/BDR election rules?" "What's the difference between Type 3 LSA and Type 5 LSA?" "Why does OSPF need area division?"

For DR/BDR, I explained the election rules based on priority and Router ID, and the role of DR/BDR in reducing the number of adjacencies. For LSA types, I detailed that Type 3 is summary LSA generated by ABR and Type 5 is external LSA generated by ASBR. For area division, I covered reducing LSA flooding scope, lowering SPF calculation overhead, and improving network stability.

BGP deep dive:

The interviewer asked: "Explain how BGP works and its path selection rules." I covered BGP neighbor establishment (Open message, Keepalive message), route updates (Update message), and route withdrawal (Withdrawn Routes), then detailed BGP's 13 path selection rules: Weight → Local Preference → locally generated routes → shortest AS-Path → Origin → MED → eBGP over iBGP → IGP Metric → and so on. Follow-up: "Why doesn't BGP use periodic updates? How does BGP handle route flapping?"

For non-periodic updates, I explained that BGP is a path vector protocol using TCP connections, with incremental route updates — periodic updates would waste bandwidth and CPU. For route flapping, I described the Route Dampening mechanism, which assigns penalty values to unstable routes and suppresses them when exceeding a threshold.

VLAN and STP:

The interviewer also asked about Layer 2 technologies: "Explain VLAN division methods and inter-VLAN routing implementation." I listed port-based, MAC-based, protocol-based, and subnet-based VLAN division methods, then described three inter-VLAN routing approaches: router-on-a-stick, Layer 3 switching, and VXLAN. Follow-up: "What's the principle of STP? What improvements do RSTP and MSTP bring?"

Round 2: Network Security + VPN (about 75 minutes)

The second-round interviewer was a technical expert in the security domain, and the questions were very deep.

Firewalls:

The first question — "Explain firewall operating modes and security policies." I covered three operating modes: packet filtering, stateful inspection, and application-level gateway, then detailed security policy configuration elements (source address, destination address, service, action, logging, etc.). Follow-up: "What's the difference between Next-Generation Firewalls (NGFW) and traditional firewalls?" I compared them from application identification, intrusion prevention, user identity authentication, and SSL decryption perspectives.

VPN technologies:

The interviewer asked: "Explain the principles and configuration of IPSec VPN." I detailed IKE Phase 1 (Main Mode/Aggressive Mode), IKE Phase 2 (Quick Mode), ESP/AH protocols, and Transport Mode/Tunnel Mode. Follow-up: "How do you solve IPSec VPN's NAT traversal problem?" I explained the NAT-T mechanism, which solves the issue of NAT devices being unable to modify ESP payloads by encapsulating ESP within UDP 4500.

There was also an SSL VPN question: "What's the difference between IPSec VPN and SSL VPN? What scenarios is each suitable for?" I compared them from protocol layer, client requirements, access granularity, and NAT traversal, noting that IPSec VPN suits site-to-site while SSL VPN suits remote access.

Network attacks and defense:

The interviewer asked: "What are common network attacks? How do you defend against them?" I listed DDoS attacks (SYN Flood, UDP Flood, HTTP Flood), ARP spoofing, DNS hijacking, and man-in-the-middle attacks, along with defense methods for each. Follow-up: "What's the traffic scrubbing solution for DDoS attacks?" I covered near-source scrubbing, blackhole routing, CDN distribution, and Anycast networking.

Network access control:

There was also a network access control question: "What's the 802.1X authentication process?" I covered the three roles of Supplicant, Authenticator, and Authentication Server, detailing EAPOL message exchange and RADIUS authentication process.

Round 3: Network Architecture Design (about 70 minutes)

The third round was with the network architecture team director, mainly assessing network architecture design capability and systems thinking.

Data center network architecture:

The interviewer gave a design question: "If you were to design the network architecture for a large data center, how would you approach it?" I described the Spine-Leaf architecture: full mesh between Spine and Leaf layers, east-west traffic doesn't need to go through the core layer, each Leaf switch has equal uplink bandwidth with no oversubscription issues. Then I detailed the BGP as Underlay, VXLAN as Overlay approach, and the advantages of EVPN as the control plane. The interviewer followed up: "What advantages does Spine-Leaf have over traditional three-tier architecture?" "How is VXLAN's VTEP implemented?" "What's the difference between EVPN Type 2 and Type 5 routes?"

WAN architecture:

The interviewer asked: "How would you design a WAN architecture for a multinational enterprise?" I described Hub-Spoke and Full-Mesh topologies, then introduced the SD-WAN approach: Underlay (Internet/MPLS/4G) + Overlay (IPSec tunnels) + Intelligent path selection (based on application, link quality, policy). Follow-up: "How does SD-WAN's intelligent path selection algorithm work?"

Network automation:

There was also a network automation question: "Are you familiar with network automation? What practices have you done?" I described Netconf/YANG, Ansible network modules, Python scripting (Netmiko/NAPALM), and SDN controllers (OpenFlow). Follow-up: "If you were to implement a network configuration automation platform, how would you design it?"

High availability design:

The interviewer's final question was about high availability: "How do you design a highly available network architecture?" I covered three levels: device-level redundancy (active-active/VRRP), link-level redundancy (link aggregation/ECMP), and path-level redundancy (routing protocol multi-path). Follow-up: "What's the difference between VRRP and HSRP?"

Key Interview Questions

OSPF:

1. OSPF working principles (areas, state machine, LSA, SPF)

2. DR/BDR election rules

3. Difference between Type 3 LSA and Type 5 LSA

4. Reasons for OSPF area division

BGP:

5. BGP working principles and 13 path selection rules

6. Why BGP doesn't use periodic updates

7. BGP route flapping handling (Route Dampening)

Layer 2 Technologies:

8. VLAN division methods and inter-VLAN routing

9. STP/RSTP/MSTP principles and improvements

Firewalls:

10. Firewall operating modes and security policies

11. Differences between NGFW and traditional firewalls

VPN:

12. IPSec VPN principles and configuration

13. IPSec VPN NAT traversal

14. Differences between IPSec VPN and SSL VPN

Security:

15. Common network attacks and defense methods

16. DDoS traffic scrubbing solutions

17. 802.1X authentication process

Architecture Design:

18. Data center Spine-Leaf architecture

19. VXLAN and EVPN solutions

20. SD-WAN WAN architecture

21. Network automation practices

22. Network high availability design

23. Difference between VRRP and HSRP

Lessons and Advice

1. Understand network protocol principles, don't just memorize concepts. Cisco's interviewers demand deep protocol understanding. For OSPF, it's not enough to recite seven neighbor states — they'll ask about state transition conditions, LSA flooding processes, and SPF algorithm details. I recommend using a packet capture tool (Wireshark) to actually capture OSPF and BGP packets for much deeper understanding.

2. Security knowledge shouldn't stay at the theoretical level. Many security questions the interviewer asks are about real-world scenarios, like IPSec NAT traversal and DDoS traffic scrubbing. These can't be understood just from reading — you need hands-on configuration and troubleshooting experience. I recommend setting up IPSec VPN in a lab environment and actually configuring and troubleshooting it.

3. Architecture design requires a holistic view. Don't just know how to configure a single device — understand the design philosophy of the entire network. Spine-Leaf architecture, SD-WAN solutions, and network automation are current hot topics in networking that you must understand deeply before the interview.

4. Cisco certifications are a plus. If you have CCNP or CCIE certification, you'll have an advantage in the interview. While not a hard requirement, certification demonstrates that you've systematically studied Cisco networking technologies, and interviewers will recognize your professional competence.

5. Reference Cisco products in your answers for bonus points. If you can reference Cisco products (like Catalyst switches, ASR routers, Firepower firewalls, etc.) when answering questions during the interview, it shows you're familiar with Cisco's product line, which is a significant advantage.

FAQ

Q: Does Cisco's network engineer interview require certifications?

A: Not a hard requirement, but having CCNP/CCIE certification is an advantage. I didn't have CCIE when I interviewed, but 2 years of project experience made up for the lack of certification.

Q: Do I need to know programming?

A: Python basics are a plus. Cisco is pushing network automation, and you may encounter Python scripting and Netconf-related questions in the interview. You can pass without programming skills, but having them opens more opportunities.

Q: Will there be lab exercises in the interview?

A: No hands-on labs, but the interviewer will give you configuration scenarios and ask you to describe your configuration approach verbally. For example, "How would you configure multi-area OSPF" or "How would you configure IPSec VPN."

Q: What does a Cisco network engineer's work involve?

A: It depends on the specific team — some do pre-sales network design, some do post-sales network implementation, and some do product technical support. I recommend understanding the specific role responsibilities before the interview.

Q: How is the compensation?

A: With 2 years of experience, Cisco network engineer compensation is above average, depending on the team and negotiation. Benefits are comprehensive, including full social insurance and supplemental medical insurance.