Network Administrator Interview Questions
They say that lightning doesn’t strike the same place twice. In my experience, that’s a little like network administrator interviews. They come in many flavors, intensity and levels. The common factor is that they all seek to find competent network administrators. In my humble opinion some techniques work well in order to highlight suitable candidates and others are about as reliable as a random number generator.
Nevertheless, whether you are an entry level candidate or an experienced consultant the basics remain the same. It is just the level of detail and understanding that changes.
Understand the business
First and foremost, a network administrator should understand that the infrastructure that he or she supports is a platform for numerous critical business systems. Because all data is carried by the network, the platform becomes a dependency for every system that requires data communication. It is important to express this understanding during your network administrator interview because it demonstrates an understanding of risk assessment and impact. The last word you want to utter as a network administrator is ‘oops’. And the employer needs to feel comfortable that you will take all precautions to avoid that or any other expressive adjectives caused by carelessness during the course of your duties.
Know the key technologies
Secondly, there are some basic concepts and technologies that are common around the world. I have always admired the Information Technology industry for its incredible application of standards in a brief period of time, relatively speaking. Concepts such as the difference between routing and switching, and the OSI model and technologies such as Ethernet and TCP/IP should be well understood from a technical standpoint. Methodologies like ITIL and Prince2 should be understood from a business standpoint.
Express yourself clearly in many levels
Thirdly, communication skills. There is a reason why network administrator job advertisements nearly always specify ‘excellent communication skills’ as part of their criteria. As a network administrator you are the interface between the acronyms and the technical jargon that almost constitutes a language in its own right and the rest of the world. Translating problems and concepts does require excellent communication skills. And demonstrating your ability to communicate at different levels is a key factor to winning the network administrator interview.
Stay up to date with emerging technologies
And last but not least, bring a professional identity to the interview. Show up in full character, talk the talk, know your hot technologies, list emerging trends and understand contemporary issues. Be prepared to converse about networking issues to a point where you hold your own opinions and be prepared to defend your position. Let me explain what I mean. Suggesting that Video over IP is a technology to look out for is a good start. Explaining that Video over IP may well be one of a number of catalysts for a renewed demand on high(er) speed broadband technologies demonstrates an understanding of the technology, its impact on link capacity as well as market drivers.
Network Administrator Interview Questions
Situational Question.
You receive an incident log that the network in SiteX is running very slow to the point of being unusable, what do you do?
Tip. Don't get immediately into the technical stuff. Explain the process that you would take to verify the problem and understand the size and scope. Also explain your communication plan, who you will notify and what form your communication will take (phone, email etc.)
Step #1. Find out if this problem is isolated to just 1 client. If it is this points to a PC problem.
Step #2. Find out the start date of the performance problem. Check that against the change control database to see if any changes were recently made that could have caused the issue.
Step #3. Check network monitoring tools for unusual traffic patterns or high latency. If you use Cisco's IPSLA - check the output from that service.
Step #4. Check carrier notifications for communications outages in the area.
Step #5. If you have applied QOS. Check the QOS policy for class drops.
Step #6. Check the interface for layer 2 (OSI) errors.
Step #7. Check routing and routing protocols, is the traffic being routed out of the main link? Is it load balanced correctly?
Step #8. Ping accross the link. Check dropped packets and latency.
Step #9. Traceroute accross the link.
Common Knowledge Questions.
What is the difference between TCP and UDP?
TCP is a reliable protocol, UDP is unreliable - it assumes that error checking is either done in the higher layers or it is not necessary. This makes UDP a very fast protocol and ideal for applications such as VOIP (Voice Over IP)
What is the difference between routing and switching?
Routing is concerned with finding the best path for a logical Layer 3 address.
Switching is a layer 2 function generally achieved in hardware that identifies next hop physical addresses.
What is the maximum distance of a UTP cable?
100m
What does QOS stand for and what does it achieve?
It stands for Quality of Service and it helps the router or switch to prioritise important traffic at times of congestion.
What are the common ports for Telnet, DNS, HTTP, HTTPS and NTP?
TCP 23, TCP and UDP 53, TCP 80, TCP 443 and UDP 123 respectively.
What is the difference between Authentication and Encryption.
Authentication is used to verify a person's identity while Encryption is used to protect the privacy of data while in transit.
What classes are the addresses 192.168.10.3, 10.1.254.200, 172.16.21.1?
Class C, Class A and Class B respectively.
Advanced Questions
What is the difference between administrative distance and metric?
Administrative distance distinguishes which routing protocol is more 'believable' while the metric is used by the routing protocol to determine a 'cost' for the shortest path.
Common Administrative distances on Cisco routers are:
Directly connected: AD 0
Static Route: AD 1
EIGRP internal route: AD 90
EIGRP external route: AD 190
OSPF: AD 110
BGP (iBGP): 200
BGP (eBGP): 20
Some common metrics:
RIP: Hops
EIGRP: Composite Metric
OSPF: COST
BGP - Best path selection algorithm
What problem presents itself when two routers perform mutual redistribution?
Routing loops can occur if redistributed routes are not filtered. Some common strategies for filtering redistributed routes are using tags, filtering by network or filtering redistributed routes (EX in EIGRP or E1, E2 in OSPF).
Explain LLQ
LLQ stands for Low Latency Queuing. It is closely associated to Priority Queuing when used in conjunction with CBWFQ.
LLQ is often in VOIP ready Quality of Service policies.
What are the three different origin types in BGP?
iBGP, eBGP and incomplete.
iBGP origin type is any BGP advertisement originating from a neighbor with the same AS number.
eBGP origin type is any BGP advertisement originating from a neighbor with a different AS number.
Incomplete origin type is any BGP advertisement that was redistributed into the BGP process.
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