40 questions with answers for candidates with 5 years of experience — 1 easy, 23 medium, 16 hard.
What interviewers expect: 5 years experience
At five years, interviewers treat you as a lead engineer. Expect design trade-offs for redundancy, failover between WAN links, network security hardening with AAA and 802.1X, wireless design and automation of repetitive tasks. You will also be asked how you review junior engineers' changes and how you communicate with managers during major incidents.
This set is weighted towards STP and EtherChannel (8), IP Services (8), Routing and OSPF (6), Wireless and Automation (5).
High availability design Security hardening Automation with Python and Ansible Team leadership
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WPA2 uses AES-CCMP; WPA2-Personal uses a pre-shared key that is vulnerable to offline dictionary attacks, and WPA2-Enterprise uses 802.1X with RADIUS. WPA3-Personal replaces PSK with SAE, which resists offline guessing and gives forward secrecy. WPA3-Enterprise offers a 192-bit mode. WPA3 requires protected management frames. Open networks can use OWE for encryption without passwords.
02
Compare Ansible, Puppet and Chef for network automation.
Ansible is agentless, uses SSH or APIs, writes playbooks in YAML and uses a push model, so it is the most popular for network devices. Puppet uses an agent, a declarative manifest language and a pull model. Chef uses Ruby-based recipes with an agent and pull model. Puppet and Chef are more common for servers. All support idempotent configuration management.
Put members in a channel group with mode active, then configure the port-channel.
interface range GigabitEthernet1/0/47 - 48
channel-group 1 mode active
interface Port-channel1
switchport mode trunk
switchport trunk allowed vlan 10,20,99
port-channel load-balance src-dst-ip
show etherchannel summary
Mark interfaces, match inside hosts with an ACL and overload the outside interface.
interface GigabitEthernet0/1
ip nat inside
interface GigabitEthernet0/0
ip nat outside
access-list 1 permit 192.168.0.0 0.0.255.255
ip nat inside source list 1 interface GigabitEthernet0/0 overload
show ip nat translations
05
Two OSPF routers are directly connected but show no neighbour at all. How do you troubleshoot?
I check that the interfaces are up and in the same subnet, then show ip ospf interface to compare area, hello and dead timers, network type and authentication. I make sure the interface is not passive and that the network statement or ip ospf area command covers it. Finally I check that no ACL blocks protocol 89 or multicast 224.0.0.5, and use debug ip ospf adj if needed.
06
What is UDLD and why is it important on fiber links?
UniDirectional Link Detection sends probes to confirm a neighbour receives traffic in both directions. On fiber, one strand can fail so a port receives but cannot send, which may make STP unblock a port and create a loop. UDLD normal mode logs and marks the port, while aggressive mode err-disables it. It is commonly enabled on fiber uplinks together with Loop Guard.
Quality of Service gives different treatment to traffic types when links are congested. Voice needs low delay (under 150 ms one way), low jitter (under 30 ms) and low loss (under 1 percent), while bulk file transfer can wait. QoS tools classify and mark traffic, queue it with priority queues for voice, and police or shape rates. Without congestion QoS has no effect.
08
Explain the NAT terms inside local, inside global, outside local and outside global.
Inside local is the private address of an inside host, for example 192.168.1.10. Inside global is the public address that represents it outside, for example 203.0.113.5. Outside global is the real address of the remote host. Outside local is how the remote host appears to inside users, usually the same as outside global unless destination NAT is used. show ip nat translations displays these columns.
09
How would you plan an IP addressing scheme for a company with 10 branches?
I would reserve a large block, for example 10.0.0.0/16, and give each branch a summarizable /20 such as 10.0.16.0/20, 10.0.32.0/20 and so on. Inside each, assign fixed ranges per role: users, voice, wireless, servers, management and WAN links. This lets each branch be summarized as one route, keeps firewall rules simple, and leaves room to grow. I keep it in an IPAM tool.
10
Syslog timestamps from different devices do not match during an incident review. What do you fix?
Clocks are not synchronised. I configure all devices to use the same internal NTP servers with ntp server, set the timezone with clock timezone IST 5 30, add service timestamps log datetime msec localtime, and enable NTP authentication. Then I check show ntp associations and show ntp status to confirm synchronisation. This makes future incident timelines reliable.
11
An internal web server must be reachable from the internet on a public IP. How do you set it up?
Use static NAT or port forwarding so the public IP maps to the server's private IP, for example ip nat inside source static tcp 192.168.1.20 443 203.0.113.10 443. Add an ACL or firewall rule allowing only port 443 inbound, put the server in a DMZ VLAN, and create a public DNS record. Ideally place a firewall or reverse proxy in front instead of exposing the server directly.
12
Your company is adding a new branch with 40 users. What network components would you propose?
A branch router or SD-WAN edge for the WAN link and internet, one or two 48-port PoE access switches for PCs, phones and access points, and wireless access points managed by a controller or the cloud. I would add a firewall function, DHCP and DNS settings, VLANs for data, voice, guest and management, and a backup 4G or broadband link for resilience.
Member interfaces must have the same speed and duplex, the same mode (access or trunk), the same access VLAN or the same native and allowed VLAN list on trunks, and compatible negotiation modes. Mismatched settings put members into suspended state. Configure settings on the port-channel interface so they are pushed to members, and verify with show etherchannel summary.
14
Tell me about a time a change you approved caused an outage. What did you do and what changed afterwards?
Situation: I approved a change that set a new root bridge priority, but the engineer applied it on the wrong switch and the campus reconverged during business hours. Task: restore service and find the gap. Action: we rolled back in ten minutes, informed users and ran a blameless review. Result: we added device name checks to the change template, peer review of exact commands and a maintenance window rule for any STP change. There was no repeat incident that year.
Allow web traffic from one subnet to a server and block the rest of that subnet.
ip access-list extended WEB-ONLY
10 permit tcp 192.168.10.0 0.0.0.255 host 10.0.0.80 eq 443
20 deny ip 192.168.10.0 0.0.0.255 10.0.0.0 0.0.0.255
30 permit ip any any
interface vlan 10
ip access-group WEB-ONLY in
show access-lists WEB-ONLY
16
How does controller-based networking differ from traditional networking?
Traditionally each device has its own control plane and is configured box by box. In controller-based or software-defined networking, a central controller holds the policy and pushes it to devices through southbound APIs such as NETCONF or OpenFlow, while applications talk to the controller through northbound REST APIs. This gives consistent configuration, automation and network-wide visibility.
CoS is a 3-bit priority field in the 802.1Q tag, values 0 to 7, used only on Layer 2 trunks. DSCP is a 6-bit field in the IP header, values 0 to 63, which survives across routed hops. Common values: EF (46) for voice, AF41 (34) for video, CS3 (24) for call signalling and default (0) for best effort. Mark close to the source and trust markings only at controlled boundaries.
18
What is SNMP and the differences between v2c and v3?
SNMP lets a manager poll devices for counters and receive traps or informs about events, using a MIB with OIDs. Polling uses UDP 161 and traps UDP 162. SNMPv2c uses a plain-text community string with no encryption. SNMPv3 adds user-based authentication and encryption with authPriv mode, so it is the version to use in production, especially on networks reachable from many teams.
Physical ports include service ports for out-of-band management, distribution system ports that connect to the switch as trunks, and redundancy ports for HA. Logical interfaces include the management interface used for CAPWAP and admin access, dynamic interfaces that map WLANs to VLANs, and a virtual interface used for web auth and DHCP relay. A LAG bundles distribution ports.
Set interface priority; higher wins and 0 never becomes DR.
interface GigabitEthernet0/0
ip ospf priority 255
! on a router that should never be DR
interface GigabitEthernet0/0
ip ospf priority 0
clear ip ospf process
Because election is non-preemptive, reset the process or the neighbours for the change to take effect.
VLAN Trunking Protocol shares VLAN definitions from a server switch to clients in the same VTP domain. Modes are server, client, transparent and off. The risk is that a switch with a higher configuration revision number can overwrite and wipe VLANs across the domain. Many networks use transparent mode or VTP version 3 with a primary server to avoid this.
22
After a core switch replacement, ping works across the campus but large file transfers and some VPN sessions hang. What do you suspect?
Suspect an MTU mismatch. Small pings fit in one frame, but full-size packets are dropped when one link or the new switch uses a smaller MTU, or when ICMP is blocked so Path MTU Discovery fails. Test with ping using a size of 1472 and the do-not-fragment bit, compare interface MTU on both ends, and check jumbo frame settings. Fix by matching MTU end to end or adjusting TCP MSS on tunnel interfaces.
23
How do you make an access switch port a routed port on a Layer 3 switch?
A routed port behaves like a router interface and does not belong to any VLAN.
interface GigabitEthernet1/0/1
no switchport
ip address 10.0.0.1 255.255.255.252
no shutdown
Routed uplinks between distribution and core avoid spanning tree on those links and allow fast routing convergence with ECMP.
24
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.
25
How do you configure inter-VLAN routing on a Layer 3 switch with SVIs?
Enable routing and create an SVI per VLAN as the gateway.
ip routing
interface vlan 10
ip address 192.168.10.1 255.255.255.0
ip helper-address 10.0.0.50
no shutdown
interface vlan 20
ip address 192.168.20.1 255.255.255.0
no shutdown
show ip route connected
Make members routed ports first, then put the IP on the port-channel.
interface range TenGigabitEthernet1/1/1 - 2
no switchport
channel-group 10 mode active
interface Port-channel10
no switchport
ip address 10.0.0.1 255.255.255.252
show etherchannel 10 summary
show ip interface brief
Multiple Spanning Tree (802.1s) maps many VLANs to a small number of STP instances, so a switch does not run one instance per VLAN as PVST does. Switches in the same MST region must share the name, revision and VLAN-to-instance mapping. It suits large networks with hundreds of VLANs and multi-vendor environments, and supports load sharing by making different instances root on different switches.
Enable per VLAN, trust uplinks and rate-limit user ports.
ip dhcp snooping
ip dhcp snooping vlan 10,20
no ip dhcp snooping information option
ip arp inspection vlan 10,20
interface GigabitEthernet1/0/48
ip dhcp snooping trust
ip arp inspection trust
interface range Gi1/0/1 - 46
ip dhcp snooping limit rate 15
show ip dhcp snooping binding
29
Your company is running out of IP addresses on a /24 user subnet. What options do you have?
Options: expand to a /23 if the adjacent block is free and update the router and DHCP scope; create a new VLAN and subnet for some users, which also limits broadcast size; shorten DHCP lease times if many guest devices hold leases; and move printers, phones and IoT devices to their own subnets. Long term, plan an addressing scheme with growth headroom and consider dual-stack IPv6.
30
What is EUI-64 and how is the interface ID generated?
EUI-64 builds a 64-bit interface ID from a 48-bit MAC. Split the MAC in half, insert FFFE in the middle and flip the seventh bit of the first byte. For MAC 0050.56AB.CDEF the ID becomes 0250:56FF:FEAB:CDEF. Routers can use ipv6 address 2001:db8:1::/64 eui-64. Hosts often prefer random privacy addresses instead, to avoid exposing the MAC.
The underlay is the physical IP network of switches and routers that provides basic reachability, for example OSPF or IS-IS on routed links. The overlay is a virtual network built on top using tunnels such as VXLAN, so logical segments can stretch across the fabric independent of physical topology. SD-Access and SD-WAN use this model, with a fabric controlling the overlay.
Dynamic Trunking Protocol negotiates whether a link becomes a trunk. Modes are dynamic auto, dynamic desirable, trunk and access. Auto plus auto stays access, while desirable with auto or desirable forms a trunk. Attackers can use DTP to form a trunk and reach all VLANs, so best practice is to hard-code access or trunk and use switchport nonegotiate on trunks.
33
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.
34
How would you influence which port becomes the root port on a switch?
The root port is chosen by lowest cost, then sender bridge ID, then sender port ID. To prefer a link I lower its cost.
interface GigabitEthernet1/0/49
spanning-tree vlan 10 cost 2
On the upstream switch you can change port priority instead.
interface GigabitEthernet1/0/1
spanning-tree vlan 10 port-priority 64
35
Two OSPF routers on the same link are stuck in EXSTART state. What is the cause and fix?
EXSTART or EXCHANGE stuck usually means an MTU mismatch, because database description packets larger than the neighbour's MTU are dropped. I compare MTU with show interfaces and set ip mtu to match on both sides. As a temporary workaround, ip ospf mtu-ignore can be used, but fixing the MTU is better. Duplicate router IDs can also cause odd adjacency behaviour, so I check those too.
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.
A voice VLAN lets an IP phone and a PC share one switch port while keeping their traffic separate. The port is an access port in the data VLAN, and the switch tells the phone through CDP or LLDP-MED to tag voice frames with the voice VLAN. This allows QoS priority for voice and separate addressing. Configure it with switchport voice vlan.
38
How do you configure a router as a DHCP relay and verify it?
Add the helper address on the client-facing interface.
interface vlan 20
ip helper-address 10.0.0.50
show ip interface vlan 20 | include Helper
debug ip dhcp server packet
The helper also forwards some other UDP broadcasts such as TFTP and DNS by default; use no ip forward-protocol udp to limit them.
Set priorities on the intended primary and secondary roots, then verify.
spanning-tree mode rapid-pvst
spanning-tree vlan 10,20 root primary
! on the backup switch
spanning-tree vlan 10,20 root secondary
show spanning-tree vlan 10
show spanning-tree root
show spanning-tree summary
40
A branch has a primary MPLS link with OSPF and a backup broadband link. How do you design failover?
Run OSPF over MPLS for the primary path. For the backup, use a floating static default route via the broadband link with AD higher than OSPF, for example 250, or run an IPsec tunnel with OSPF at a higher cost. Track the primary with IP SLA if the provider does not drop the link on failure, so the backup activates even when the interface stays up.
Frequently asked questions
What is asked in CCNA interviews for 5 years of experience?
At five years, interviewers treat you as a lead engineer. Expect design trade-offs for redundancy, failover between WAN links, network security hardening with AAA and 802.1X, wireless design and automation of repetitive tasks. You will also be asked how you review junior engineers' changes and how you communicate with managers during major incidents.
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 5 years of experience?
Focus first on High availability design, Security hardening, Automation with Python and Ansible, Team leadership.