Network Engineer exam mistakes to avoid

Updated 15 September 2026 · 3 min read

Candidates for engineering posts usually know a lot, and that is part of the problem. They recognise familiar terms in an option and choose it without checking whether it fits the scenario. The mistakes below are the ones experienced candidates make, and each has a simple habit that prevents it.

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Applying route selection in the wrong order

The most damaging engineering error is checking administrative distance before prefix length. A candidate sees one route from OSPF and another from a static entry and picks the static route, forgetting that the OSPF route is more specific for the destination. Always find the longest matching prefix first. Distance and metric only break ties between routes of the same length.

Missing a requirement in a design question

Design stems hide their key requirement in a short phrase: "without additional hardware", "must survive a single link failure", "minimise cost". An option that is technically excellent but ignores that phrase is wrong. Before looking at the options, write down every requirement and constraint in the stem.

Overlapping subnets in a VLSM plan

Allocating subnets in the order the sites are listed, rather than largest first, often leaves gaps that later requirements cannot use, or produces overlaps when a candidate squeezes a subnet into space already taken. Sort first, allocate in order, and check each network address against the previous broadcast address.

Confusing spanning tree roles

Candidates often assume the fastest or most central switch becomes the root bridge. Without configuration, the switch with the lowest bridge ID wins, which depends on priority and then on MAC address, so an old switch in a cupboard can end up as root. Questions test exactly this surprise.

Misunderstanding first-hop redundancy

A frequent mistake is believing HSRP or VRRP balances traffic across both routers by default. In their standard form, one router is active and the other waits. Another is forgetting that without preemption, a recovered primary router does not take back the active role on its own.

Over-engineering the answer

Engineers enjoy thorough solutions, but written questions often ask for the simplest change that meets the need. If a single static route solves a small branch's routing, deploying a dynamic protocol across the network is not the expected answer. Match the scale of the solution to the scale of the problem described.

Reading OSPF cost as hop count

A path through fewer routers is not automatically the one OSPF chooses. OSPF adds up the cost of every outgoing interface along a path, and cost follows interface bandwidth. Three fast links can cost less than one slow link, so a longer path through gigabit interfaces beats a shorter one with a slow WAN hop. Remember too that with the default reference bandwidth, every interface at 100 Mbps or faster gets the same cost, which is why engineers often raise the reference value on modern networks.

Questions where these mistakes cost marks

A multi-access OSPF network needs to elect a DR and BDR. Which router attribute is evaluated first to determine the DR?
1. Highest Router ID
2. Highest Loopback IP
3. Highest OSPF Priority
4. Highest Physical IP
Show the answer
C. Highest OSPF Priority
OSPF DR election first looks at interface Priority (default is 1). If there is a tie, it breaks the tie using the highest Router ID.
Original practice question, written against published exam objectives. Open this question
An admin notices a routing loop forming between three routers running RIPv2. Which mechanism prevents routing loops by not advertising a route back on the interface it was learned from?
1. Route Summarization
2. Poison Reverse
3. Split Horizon
4. Holddown Timers
Show the answer
C. Split Horizon
Split horizon is a distance vector loop prevention mechanism that dictates a route cannot be advertised back out the interface it was learned on.
Original practice question, written against published exam objectives. Open this question
A router needs to forward a packet but its ARP cache is empty for the destination IP on a directly connected subnet. What does the router do next?
1. Sends an ICMP unreach
2. Broadcasts an ARP request for the destination IP MAC address
3. Drops the packet
4. Sends the packet to the default gateway
Show the answer
B. Broadcasts an ARP request for the destination IP MAC address
Before encapsulating a packet into a frame for a local subnet, a device must know the destination MAC address. It uses an ARP request to find it.
Original practice question, written against published exam objectives. Open this question

Common questions

How do I stop recognising terms and answering too fast?

Force a short pause. Before choosing, name the requirement in the stem that the option meets. If you cannot name one, look again at the other options.

Which topic should I drill hardest?

Route selection, because its rules feel familiar and are easy to apply in the wrong order. Work through routing tables until the sequence of prefix, distance and metric is automatic.

Should I change answers during review?

Only with a concrete reason, such as a requirement you missed or a prefix you misread. Engineering candidates often talk themselves out of a correct design because a more complex option looks more professional.

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