40 questions with answers for candidates with 1 year of experience — 24 easy, 16 medium.
What interviewers expect: 1 year experience
With one year of experience, usually in support or a NOC, interviewers expect real ticket stories. Questions cover interface errors, err-disabled ports, VLAN and trunk issues, DHCP failures and OSPF neighbour problems. Explain how you troubleshoot step by step, which commands you run, and how you escalate with clear notes and evidence.
This set is weighted towards IP Addressing and Subnetting (8), Network Fundamentals (6), Network Security (6), Wireless and Automation (5).
Layer 1 and 2 troubleshooting DHCP and NAT issues OSPF neighbour states Incident handling
Solved at this levelStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
✓ 3 questions free • Unlock the rest in seconds
01
A Python script gets 401 errors when calling a controller REST API. What do you check?
401 Unauthorized means authentication failed. I check the username and password or API key, whether a token must first be requested from an auth endpoint and sent in a header like X-Auth-Token, whether the token expired, and the exact header name and format. I test the same call with curl or an API client to separate script bugs from credential issues.
RFC 3021 allows /31 on point-to-point links because there is no need for network and broadcast addresses when only two devices exist. A /31 gives exactly two usable addresses, saving half the space of a /30. It is common on ISP and data center fabric links. Both ends must support it; on IOS you simply configure ip address 10.0.0.0 255.255.255.254.
Switches exchange BPDUs and the one with the lowest bridge ID becomes root. The bridge ID is the priority (default 32768 plus the VLAN number in PVST) followed by the MAC address, so if priorities tie, the lowest MAC wins. Because the oldest switch often has the lowest MAC, you should set priority manually, for example spanning-tree vlan 10 root primary on the core.
802.11n (Wi-Fi 4) works on 2.4 and 5 GHz with MIMO. 802.11ac (Wi-Fi 5) is 5 GHz only with wider channels and MU-MIMO downlink. 802.11ax (Wi-Fi 6 and 6E) adds OFDMA, BSS colouring and target wake time, and 6E adds the 6 GHz band. 802.11be (Wi-Fi 7) adds 320 MHz channels and multi-link operation. India opened part of the 6 GHz band for indoor use.
05
A newly configured router interface rejects the IP 192.168.5.64/26. Why?
With a /26 the block size is 64, so 192.168.5.64 is the network address of the subnet .64 to .127 and cannot be assigned to a host or interface. IOS reports a bad mask or overlap error. Use a usable address such as 192.168.5.65 instead. Similarly .127 would be the broadcast. Always calculate the subnet boundary before configuring addresses.
The client sends a SYN with an initial sequence number. The server replies with SYN-ACK, acknowledging the client sequence and sending its own. The client sends an ACK and the session is established. Closing uses FIN and ACK from each side. A firewall that sees repeated SYNs without the final ACK may be looking at a SYN flood attack.
07
What is AAA and the difference between TACACS+ and RADIUS?
AAA stands for Authentication (who you are), Authorization (what you can do) and Accounting (what you did). TACACS+ uses TCP 49, encrypts the whole payload and separates authentication from authorization, so it is preferred for device administration with per-command control. RADIUS uses UDP 1812 and 1813, encrypts only the password and is used for network access such as 802.1X and VPN.
DHCP uses the DORA process: the client broadcasts Discover, the server answers with an Offer, the client broadcasts a Request for that offer, and the server confirms with an Acknowledgement. The lease includes IP, mask, gateway, DNS servers and lease time. Clients renew at 50 percent of the lease. DHCP uses UDP ports 67 for the server and 68 for the client.
09
How do you handle a change that must be done during a night window?
Situation: we had to migrate 30 access ports to new VLANs overnight. Task: finish with zero business impact. Action: I prepared configs in advance, got peer review, scheduled a rollback, informed the helpdesk and tested each floor with a laptop after changes. Result: the migration finished an hour early, morning tickets were zero, and the plan became our template for later VLAN changes.
DHCP Discover is a broadcast, and routers do not forward broadcasts, so a central DHCP server cannot hear clients on other subnets. A relay agent, configured with ip helper-address on the gateway interface, converts the broadcast into a unicast to the server and adds the gateway IP so the server picks the right scope. This lets one server serve many VLANs.
In 2.4 GHz each channel is about 22 MHz wide but channels are only 5 MHz apart, so most adjacent channels overlap and interfere. Channels 1, 6 and 11 are the only three non-overlapping channels, so neighbouring access points should alternate between them. The 5 GHz band has many more non-overlapping 20 MHz channels, which is why it is preferred for capacity.
12
What is the difference between standard and extended ACLs?
Standard ACLs (1 to 99 and 1300 to 1999) match only the source IP, so they should be placed close to the destination. Extended ACLs (100 to 199 and 2000 to 2699) match protocol, source, destination and ports, so they can be placed close to the source to drop unwanted traffic early. Named ACLs work for both and allow editing individual lines by sequence number.
When a host needs the MAC for an IP on its subnet, it broadcasts an ARP request and the owner replies with a unicast ARP reply; the result is cached. For remote destinations the host ARPs for its default gateway. A gratuitous ARP is an unsolicited announcement of a host's own IP-to-MAC mapping, used after failover, for example by HSRP, to update switch and host tables.
Rapid PVST+ runs 802.1w per VLAN. It uses a proposal and agreement handshake on point-to-point links so ports move to forwarding in under a second, instead of waiting on timers. It has port roles root, designated, alternate and backup, and states discarding, learning and forwarding. Alternate ports provide an instant backup root port. It is the default on most modern IOS switches.
Variable Length Subnet Masking means using different prefix lengths inside one address block, so each subnet is sized to its need. You allocate the largest subnets first, then smaller ones, to avoid overlaps. For example, from 10.1.1.0/24 you could give a 100-user LAN a /25, a 50-user LAN a /26, and point-to-point WAN links /30 or /31. It saves address space compared with fixed-length subnetting.
16
Explain password security best practices for network devices.
Use enable secret, not enable password, so the hash is strong (type 8 or 9 on modern IOS). Create local users with secret, enforce minimum length with security passwords min-length, and use AAA with TACACS+ for central control. Disable Telnet, use SSH v2, set exec-timeout on lines, and add login block-for to slow brute force. Avoid type 7 passwords, which are easily reversed.
17
Describe a time you made a configuration mistake. How did you handle it?
Situation: I applied a /25 mask instead of /24 on a core VLAN interface during a change window. Task: fix it fast and own the error. Action: monitoring alerted on half the users, I checked my change, corrected the mask within ten minutes and informed my lead immediately. Result: impact was short, and I added a peer-review step and a pre-written rollback to every change plan afterwards.
18
What is a collision domain and a broadcast domain?
A collision domain is a segment where two devices sending at the same time can collide; every switch port is its own collision domain, and with full duplex collisions do not occur. A broadcast domain is the set of devices that receive a Layer 2 broadcast. Routers and VLANs separate broadcast domains, so one VLAN equals one broadcast domain.
Areas limit how far detailed LSAs flood, so SPF runs on a smaller database and changes in one area do not force recalculation everywhere. Area 0 is the backbone and every other area must connect to it, directly or through a virtual link. Routers connecting areas are ABRs and can summarize routes between areas. Single-area OSPF is the focus of CCNA.
20
What are DR and BDR in OSPF and why are they needed?
On multi-access networks such as Ethernet, a full mesh of adjacencies would create too many LSAs. OSPF elects a Designated Router and Backup DR; other routers (DROTHERs) form full adjacency only with them and stay 2-Way with each other. The highest interface priority wins, then highest router ID; priority 0 means never DR. Election is non-preemptive, so a new better router does not take over.
21
What is NTP and why does time matter on network devices?
NTP synchronises clocks over UDP 123 using a hierarchy of strata, where stratum 1 servers connect to an atomic or GPS clock. Correct time matters because syslog and incident timelines must line up across devices, certificates and authentication protocols check time, and logs are legal evidence. Devices can be NTP clients, servers or peers, and NTP authentication protects against fake time sources.
22
How do you create a VLAN and assign an access port?
Create the VLAN, name it, and set the port mode and VLAN.
vlan 20
name SALES
interface GigabitEthernet1/0/5
switchport mode access
switchport access vlan 20
switchport voice vlan 150
spanning-tree portfast
show vlan brief
23
How do you quickly calculate how many subnets and hosts a mask gives?
show ip interface brief gives status and IP of all interfaces, and show interfaces gives counters and duplex.
Router# show ip interface brief
Router# show interfaces GigabitEthernet0/1
Router# show interfaces status
Router# clear counters GigabitEthernet0/1
Status up/down means Layer 1 is up but Layer 2 keepalives fail; administratively down means the interface is shut.
25
How do you configure PortFast and BPDU Guard on access ports?
Apply per port or globally for all PortFast ports.
interface range GigabitEthernet1/0/1 - 46
switchport mode access
spanning-tree portfast
spanning-tree bpduguard enable
! or globally
spanning-tree portfast default
spanning-tree portfast bpduguard default
errdisable recovery cause bpduguard
errdisable recovery interval 300
A switch reads the source MAC of each incoming frame and records it with the port in the MAC address table. For forwarding it looks up the destination MAC: a known unicast goes out one port, while unknown unicast, broadcast and most multicast frames are flooded in the VLAN. Entries age out after 300 seconds by default. Check with show mac address-table.
27
Two hosts are in the same VLAN but one has mask /24 and the other /25. What problems may occur?
Each host decides local or remote using its own mask. With 192.168.1.10/24 and 192.168.1.200/25, the first thinks .200 is local and ARPs directly, but the second sees .10 as remote and sends replies to its gateway. Traffic can become one-way or asymmetric and may break completely if the gateway does not route it back. Fix the mask so both match the design.
Syslog has eight levels from 0 to 7: emergency, alert, critical, error, warning, notification, informational and debugging. A useful memory line is Every Awesome Computer Engineer Will Need Ice cream Daily. Setting logging trap 6 sends levels 0 to 6 to the server. Syslog uses UDP 514 by default, and logs should carry timestamps with service timestamps log datetime msec.
29
Explain root port, designated port and blocked port.
Every non-root switch picks one root port, its best path to the root based on lowest root path cost, then lowest sender bridge ID, then lowest sender port ID. On each segment, the switch with the best path becomes the designated port and forwards. Every remaining port becomes non-designated and blocks. All ports on the root bridge are designated.
30
What is the structure of an IPv6 address and how is it shortened?
IPv6 addresses are 128 bits written as eight groups of four hex digits separated by colons. Leading zeros in each group can be dropped, and one run of consecutive all-zero groups can be replaced with :: only once. For example 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1. A typical LAN uses a /64 prefix.
31
What is a wildcard mask and how is it used in ACLs?
A wildcard mask tells the router which bits must match: 0 means must match and 1 means ignore. It is often the inverse of the subnet mask, so 192.168.1.0 0.0.0.255 matches the whole /24. host 10.1.1.1 equals 0.0.0.0 and any equals 255.255.255.255. Wildcards are also used in OSPF network statements. Calculate it by subtracting the mask from 255.255.255.255.
32
What is the difference between running-config and startup-config?
running-config lives in RAM and holds the active configuration, so every change applies immediately but is lost on reload. startup-config is stored in NVRAM and is loaded at boot. You save with copy running-config startup-config or write memory. To roll back unsaved changes you can reload without saving, which is why many engineers use reload in 10 before risky remote changes.
33
How do you verify VLANs, trunks and MAC learning on a switch?
These show commands cover most VLAN troubleshooting.
show vlan brief
show interfaces trunk
show interfaces Gi1/0/5 switchport
show mac address-table vlan 20
show mac address-table address 0050.56ab.cdef
show interfaces status
They show VLAN membership, allowed and active VLANs on trunks, native VLAN and where each MAC was learned.
34
What are the key elements of a security program for a network?
A security program combines user awareness training, physical access control, and technical controls such as strong authentication, firewalls, IPS, VPNs and patching. It is built around the CIA triad: confidentiality, integrity and availability. Policies define what is allowed, and logging and monitoring detect problems. CCNA focuses on device hardening, ACLs, Layer 2 security, AAA, VPNs and wireless security.
35
A user's port keeps going into err-disabled state. How do you investigate?
I run show interfaces status err-disabled and show errdisable recovery to see the cause, such as psecure-violation, bpduguard or link-flap. If it is port security, the user may have plugged in a small switch or a docking station with an extra MAC, so I check show port-security interface. I fix the cause, then shut and no shut the port, or set the max MAC count appropriately.
Control and Provisioning of Wireless Access Points is the protocol between lightweight APs and the WLC. It builds two tunnels: a control tunnel on UDP 5246, encrypted with DTLS, and a data tunnel on UDP 5247 that carries client traffic in central switching mode. In FlexConnect or local switching mode, client data is switched locally at the AP instead of being tunnelled.
The SSID is the network name users see. The BSSID is the MAC address of the access point radio serving a BSS, the basic service set of one AP and its clients. An ESS is multiple BSSs sharing the same SSID connected by a wired distribution system, so clients can roam between APs. Each AP can broadcast several SSIDs, each with its own BSSID.
A VLAN is a logical Layer 2 broadcast domain created on a switch, independent of physical location. VLANs separate users, voice, guests and servers, reduce broadcast traffic, and improve security because traffic between VLANs must pass through a router or Layer 3 switch where it can be filtered. For example, finance and guest Wi-Fi users can share switches but stay isolated.
Example line:
O 10.2.2.0/24 [110/20] via 10.0.12.2, 00:05:12, GigabitEthernet0/0
O is the source (OSPF), 10.2.2.0/24 the prefix, 110 the administrative distance and 20 the metric, 10.0.12.2 the next hop, then the age and exit interface. C is connected, L local host route, S static, D EIGRP and an asterisk marks the candidate default.
Situation: in college I set up the hostel Wi-Fi and LAN for 120 students. Task: fix constant drop-outs. Action: I learned VLANs, channel planning and DHCP, then practiced every CCNA topic in Packet Tracer and GNS3. Result: complaints dropped sharply, and I realised I enjoy solving real problems that affect many people. Networking underpins cloud, security and automation, so it gives me a long and growing career path.
Frequently asked questions
What is asked in CCNA interviews for 1 year of experience?
With one year of experience, usually in support or a NOC, interviewers expect real ticket stories. Questions cover interface errors, err-disabled ports, VLAN and trunk issues, DHCP failures and OSPF neighbour problems. Explain how you troubleshoot step by step, which commands you run, and how you escalate with clear notes and evidence.
How many questions should I practise before a CCNA interview?
Work through all 40 questions here, then the company-wise sets for the employers you are targeting. Quality matters more than count: be able to explain why each answer is right.
Which topics matter most for 1 year of experience?
Focus first on Layer 1 and 2 troubleshooting, DHCP and NAT issues, OSPF neighbour states, Incident handling.