Are you looking for job as a network engineer/network administrator? Or are you thinking to leave your current position for a new job as a network engineer/administrator with a new company in a routed LAN/WAN environment?

If you answered yes to either of those questions, then this article is for you and any of described technologies and questions may be asked of you during the interview!

Network Engineer is a higher-level position, often with a “junior” or “senior” prefix. The major responsibility of a network engineer is to determine “how to implement technologies” in a routed LAN/WAN environment. They design and implement both the hardware and software technologies needed for a computer network. They have high-level technical skills in local area networks (LANs) or wide area networks (WANs).

Network Administrator is responsible for the smooth, efficient, and secure operation of computer networks. In general, they configure and administer existing networks rather than designing networks from the beginning. They play a very challenging role in a routed LAN/WAN environment, including customization of the network as per the organization’s needs, such as adding software and hardware, performance monitoring, troubleshooting, logging errors, backing up and restoring data, assigning permissions to users, and helping users with network issues.

Before facing any Interview for network engineer/administrator position, make sure that you have enough knowledge on below technologies.

General network concepts:

  • Data communication and transmission techniques
  • Fundamentals of OSI and TCP/IP model
  • Router’s basic operations (startup, NVRAM, flash/IOS backup and recovery)

IP addressing and summarization:

  • IP address classes
  • Classful and classless IP addresses
  • IP subnetting
  • Understating wild card masks
  • CIDR,FLSM,VLSM
  • IPv6 fundamentals

Routing:

RIP
  • Difference between RIPv1,RIPv2 and RIPng
  • Passive Interface
  • RIP Timers
  • RIP AD and Multicast Address
  • Split Horizon and Route Poisoning
EIGRP
  • Auto and Manual Summarization
  • Neighborship Conditions
  • Passive Interface
  • Split Horizon
  • Authentication
  • EIGRP Stub Routing and Stuck in Active
  • Equal and Unequal Load Balancing
  • EIGRP ADs and Multicast Address
OSPF
  • OSPF Area Types
  • OSPF Neighborship Conditions
  • Concepts of ABR and ASBR Router
  • DR/BDR Fundamentals and Election
  • OSPF Times and Authentications
  • OSPF Summarizations (Inter-Area, External, and Default Info Originate)
  • OSPF AD and Multicast Addresses
  • OSPF Network and LSA Types
BGP
  • BGP Fundamentals – Why and When to Use BGP?
  • BGP States and Message Types
  • BGP Neighborship Conditions
  • iBGP and eBGP
  • BGP Summarization
  • Use of Update Source, eBGP-Multi-Hop, Next-Hop-Self Commands
  • BGP Path Attributes
  • BGP Synchronization and Split-Horizon Rule
  • BGP Address Families
  • BGP Communities
MPLS
  • MPLS Fundamentals – IP CEF, LIB, LFIB
  • LDP and TDP
  • P, PE and CE Routers
  • PUSH,POP,SWAP Functions
  • PHP – Penultimate Hop Popping
  • BGP – VPN
  • MPLS over ATM /Frame Relay
QoS
  • QoS Models and Tools
  • Difference between L2 and L3 Queues
  • Characteristics of CoS, ToS, IPP, DSCP AF and EF
  • Class Maps and Policy Maps
Route Filtering
  • Access-List Fundamentals
  • Route-Maps
  • Prefix-Lists
  • Distribute-Lists
  • Filter-Lists
IP Services and Network Securities
  • First-Hop Redundancy Protocols (HSRP,VRRP,GLBP)
  • Network Address Translation (Static, Dynamic, PAT)
  • Network Time Protocols, Syslog Server, SNMP
  • Basics of VPNs (IPsec, Site to Site , DMVPN, Remote VPN)
  • IP Multicast Routing (IGMP,PIM SM/DM, MSDP)
  • Policy-Based Routing
  • IP SLA

My Best Questions for an Interview of Network Engineer/Network Administrator: All of the questions below are very common and must be prepared for before facing any interview for the data-WAN environment.

1. What is a router? Or define the basic requirements of a router?

Answer: A router is a layer 3 network device used to establish communication between different networks. Basic roles performed by a router are:

  • Inter-network communication
  • Best path selection
  • Packet forwarding
  • Packet filtering
2. What is the use of routing? or Why we use routing?

Answer: By default, a router provides inter-network communication only for directly connected networks. To establish communication between indirectly connected networks, we require ROUTING. We can use static or dynamic (IGP or EGP) routing, according to topology requirement.

3. Define the criteria for best path selection of a router?

Answer: A router’s routing table contains only best route. To select a route as best, a router considers the following parameters;

  • Longest prefix match
  • Minimum AD (administrative distance)
  • Lowest metric value

If all listed parameters are the same, then it would perform equal cost load balancing.

4. Define “stuck in active.”

Answer: If a successor route (best route) fails, then the router sends a query message to its neighbor demanding a feasible successor (back-up route) and a query received by the router may be forwarded to other neighbors that could lead to a loop, as well. The wait for the response of query message is called “stuck in active” (SIA).

5. Can we use OSPF without backbone area?

Answer: Yes, but it will be limited to intra-area (same area) communication. By default, Inter-area communication is not possible without backbone area.

6. What do you mean by OSPF transit area ?

Answer: A transit area is the area that has a virtual link connecting two or more ABRs attached to this area.

7. What is the difference between an OPPF neighbor and an adjacent neighbor?

Answer: Neighbors are the routers that are in the same area and exchange hello packets, but not LSA information. Adjacent routers are routers that have fully exchanged their LSA information and are stable.

If OSPF state is in 2WAY/DROTHER, it means a neighbor relationship and, if the state is FULL/DR or FULL/BDR, it means that the adjacency is formed.

8. BGP neighborship is not coming up. Please define the various steps to troubleshoot it.

Answer: To troubleshoot BGP, first we need to check neighbor state using “show ip bgp summary.” If the state is Idle, it means that the peer address or AS is not defined properly; if the state is Active, it means that TCP port 179 is not open, the peer is not reachable, network congestion, or BGP misconfiguration.

Common neighbor stability problems of BGP

  • Misconfigured neighbor’s IP address and AS number
  • Reachability issues when interfaces other than directly connected interfaces are used while peering (update-source issue).
  • Authentication must be properly implemented (if configured)
  • Router-ID must be unique

You can get more information from my BGP Tshoot article.

9. What is route reflector and why it is required?

Answer: Route reflector is a solution for BGP split horizon. The rule says “prefix learned from an iBGP neighbor will not be advertised to another iBGP neighbor.”
To overcome this situation, we have multiple options:

  1. Make your network a full mesh
  2. Route confederation
  3. Confederation

Route reflector is something like a central point acting as a route reflector server: Rather than peering with every iBGP router in a full mesh, it makes IBGP neighbors as route reflector clients to overcome the split horizon issue.

10. What is the difference between standard and extended ACL?

Answer: Standard ACLs are source-based, whereas extended ACLs are source- and destination-based. Standard ACLs can only filter layer 3 network traffic, while extended ACLs can be used to filter layer 3 and layer 4, as well.

11. What is the use of distribute-list?

Answer: To filter a routing database, we use distribute-list, which can be applied over most routing protocols. This means that, If you don’t want any specific network in your routing table, then you can use distribute-list.

12. MPLS works on which layer?

Answer: MPLS operates between layer 2 and layer 3, so it is sometimes called layer 2.5.

13. What is penultimate hop popping (PHP) and what is its use?

Answer: PHP is the technique for removing the (POP) MPLS label before the egress router. The MPLS label on a switched packet is popped by either the egress router or the penultimate router, depending on your configuration. If you decide to use penultimate hop popping, you essentially terminate the LSP one hop earlier. The MPLS labels are popped by the routers that connect to the egress router, rather than all of them being popped by the same egress router.

14. What is the difference between layer 2 and layer 3 QoS?

Answer: L2-QoS is at the MAC layer and can be applied by using CoS (class of service) filed in the VLAN header. This will be used to prioritize traffic. Later, a QOS scheduler can use the COS filed to qualify the traffic into different QOS queues.

L3 QOS is required for IP level classification; it can be achieved through ToS (type of service) priority values—IPP (3-bit), DSCP (6-bit), which can be set in the TOS field of the IP header. This TOS will later be used by scheduling process to achieve QOS.

L2 queues are hardware-based, while L3 queues are software-based. That’s why we can modify L3 queues to meet our requirements.

The questions above are very tricky and important from the standpoint of clearing any interview for a network engineer/administrator position. It is not possible for anyone to explain all kinds of questions, but you can get more frequently asked interview questions for network engineer/network administrator positions from the download link posted here. If you find any difficulty in answering any questions, then you can write me @ Comment section.

Tips for Preparing for an Interview

  • Study: Before an interview, take a quick recap of relevant technologies.
  • Updated Resume: Read your resume through; don’t copy and paste your resume. You must be aware of your strengths and weaknesses.
  • Professional Certifications: One of the best ways to prove the technical skills mentioned in your resume is through certifications. This gives a new employer an easy way to understand your knowledge level.
  • Updated LinkedIn Profile: Update your LinkedIn profile regularly; make sure that your work experience, qualifications, and project details match with your resume.

This article is the output of lots of my research and extensive work experience. With this article I have tried to help/guide candidates about interview preparation for getting a network engineer/network administrator position in a routed LAN/WAN environment.

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