A network engineer designs, implements and troubleshoots routed and switched networks across campuses, branches and data centers. Work includes OSPF and static routing design, STP and EtherChannel tuning, HSRP gateways, VLSM addressing plans, change windows and root cause analysis for major incidents. Engineers with two to five years of experience and a CCNA, often working towards CCNP, are expected to lead changes and guide support engineers.
OSPF and static routing STP and EtherChannel design IP address planning Change management Root cause analysis
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01
What is the difference between an access port and a trunk port?
An access port belongs to one VLAN and carries untagged frames, typically to PCs, printers or servers. A trunk port carries traffic for many VLANs between switches, routers or hypervisors, adding an 802.1Q tag to identify each frame's VLAN, except the native VLAN which is sent untagged. Access ports can also carry a separate voice VLAN for IP phones.
02
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.
Global unicast (2000::/3) is publicly routable. Unique local (fc00::/7, in practice fd00::/8) is like private IPv4. Link-local (fe80::/10) is auto-created on every IPv6 interface and used for neighbour discovery and routing next hops. Multicast is ff00::/8, for example ff02::1 all nodes and ff02::2 all routers. Anycast is a unicast address shared by several devices. IPv6 has no broadcast.
04
After adding a new VLAN, users on one access switch cannot use it but others can. Why?
The most likely cause is the trunk to that switch does not allow the new VLAN, because someone configured an explicit allowed list. Check show interfaces trunk and add it with switchport trunk allowed vlan add 50, not without add, which would replace the list. Also confirm the VLAN exists on that switch with show vlan brief, especially if VTP is transparent.
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.
Configure the same group and virtual IP on both switches, with higher priority on the active one.
interface vlan 10
ip address 192.168.10.2 255.255.255.0
standby version 2
standby 10 ip 192.168.10.1
standby 10 priority 110
standby 10 preempt
show standby brief
Hosts use 192.168.10.1 as their gateway.
08
Tell me about a time you found a security issue and reported it.
Situation: during a routine audit I noticed SNMP v2c with the default community public on several access switches. Task: close the gap without disrupting monitoring. Action: I reported it to my lead with evidence, proposed SNMPv3 with an ACL, tested with the monitoring team and rolled out in two batches. Result: the risk was closed in a week and SNMP checks were added to our device baseline.
09
What are the default STP path costs for common link speeds?
With short (802.1D revised) costs: 10 Mbps is 100, 100 Mbps is 19, 1 Gbps is 4 and 10 Gbps is 2. With the long pathcost method, 1 Gbps is 20000 and 10 Gbps is 2000, which distinguishes higher speeds. Root path cost is the sum of costs on ports toward the root, and the lowest total wins when choosing root ports.
10
Tell me about a time you led a network migration or a major change.
Situation: our office had to move 300 users from old access switches to a new stack in one weekend. Task: I owned the plan. Action: I documented every port, pre-built configs, ran a pilot on one floor, wrote a rollback plan and kept a call bridge open with the desktop team. Result: we finished two hours early with only three port issues, fixed on Monday morning. I learned that a tested rollback plan gives the whole team confidence.
Use one trunk link and a subinterface per VLAN with 802.1Q encapsulation.
interface GigabitEthernet0/0
no shutdown
interface GigabitEthernet0/0.10
encapsulation dot1Q 10
ip address 192.168.10.1 255.255.255.0
interface GigabitEthernet0/0.20
encapsulation dot1Q 20
ip address 192.168.20.1 255.255.255.0
The switch port facing the router must be a trunk.
Set a router ID, enable OSPF on interfaces and make user ports passive.
router ospf 1
router-id 1.1.1.1
auto-cost reference-bandwidth 100000
passive-interface GigabitEthernet0/1
network 10.0.12.0 0.0.0.3 area 0
network 192.168.1.0 0.0.0.255 area 0
default-information originate
Or per interface: ip ospf 1 area 0
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 you worked with a difficult end user during a ticket.
Situation: a senior manager was angry that his laptop lost network access before a client call. Task: fix it fast and calm him down. Action: I listened, gave a clear time estimate, found the port had been moved to a wrong VLAN in a recent change, corrected it and confirmed the call worked. Result: he was back online in ten minutes, and I added a port description check to our change process.
15
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.
16
Traffic is taking a slow path because an old access switch became root bridge. How do you fix it?
The old switch has the lowest MAC with default priority, so it won the election. I set the core or distribution switch as root with spanning-tree vlan 1-4094 priority 4096, or root primary, and the secondary with 8192. I add Root Guard on distribution downlinks so access switches can never become root again. Then I verify with show spanning-tree root.
17
A trunk between two switches is not forming. What would you check?
I check show interfaces trunk and show interfaces switchport on both sides. Common causes are mismatched modes (both dynamic auto), one side hard-coded as access, encapsulation not set on older switches, or the port being err-disabled. Native VLAN mismatch and allowed list differences do not stop the trunk but break traffic. Best fix is switchport mode trunk on both ends with nonegotiate.
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
Cost equals reference bandwidth divided by interface bandwidth. The default reference is 100 Mbps, so FastEthernet, Gigabit and 10 Gigabit all get cost 1, which hides the faster links. Change it with auto-cost reference-bandwidth 100000 on every router so 1G becomes 100 and 10G becomes 10. You can also set ip ospf cost directly on an interface. Path cost is the sum of outgoing interface costs.
A private VLAN splits one VLAN into a primary VLAN and secondary VLANs. Isolated ports can talk only to promiscuous ports such as the gateway, while community ports can talk to each other and to promiscuous ports. It is used in hosting, DMZs and hotel networks to isolate customers on the same subnet without creating many subnets. It requires VTP transparent mode or VTP v3.
21
What is the difference between a two-tier and a three-tier campus design?
A three-tier design has access, distribution and core layers: access connects users, distribution aggregates access switches and applies policy, and the core provides fast transport between distribution blocks. A two-tier or collapsed-core design merges core and distribution, which suits small and medium campuses with fewer buildings. Three-tier scales better when there are many distribution blocks.
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.
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.
24
A broadcast storm brought the network down after someone connected a cable between two wall ports. How do you prevent this?
Two wall ports patched to the same switch formed a loop. To stop it now, I find the port with rising broadcast counters and shut it. To prevent repeats, enable PortFast with BPDU Guard on all access ports, keep Rapid PVST+ running everywhere, add storm-control broadcast thresholds on access ports and enable errdisable recovery with a sensible interval.
25
What is a Layer 3 switch and how is it different from a router?
A Layer 3 switch forwards packets between VLANs in hardware using SVIs or routed ports, giving very high throughput with many Ethernet ports. A router usually has fewer ports but richer WAN features such as NAT, VPN, deep QoS, varied WAN interfaces and large routing tables. In campuses, Layer 3 switches handle inter-VLAN routing while routers or firewalls handle the edge.
26
An EtherChannel shows members in suspended state. What do you check?
Suspended (s) usually means a configuration mismatch between members or with the far end. I compare speed, duplex, trunk mode, native VLAN and allowed VLANs on each member and the port-channel with show run interface, check LACP modes on both sides, and look at logs. I fix the config on the port-channel, then shut and no shut members so they rejoin. show etherchannel summary should show P flags.
27
An SVI shows status down/down even though it is configured. What could be wrong?
An SVI comes up only if the VLAN exists in the VLAN database and at least one access or trunk port in that VLAN is up and forwarding, and the SVI is not shut down. So I check show vlan brief for the VLAN, check that at least one port in it is connected, check trunks allow it, and run no shutdown on the SVI. On Layer 3 switches, ip routing must be enabled for routing.
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.
29
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.
30
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.
Routers send hellos to 224.0.0.5. They become neighbours if area ID, subnet and mask, hello and dead timers, authentication and stub flags match, and the router IDs are unique. States go Down, Init, 2-Way, ExStart, Exchange, Loading and Full. In ExStart and Exchange they agree master and slave and swap database descriptions, which is where an MTU mismatch causes a stuck adjacency.
A floating static route is a backup static route with a higher administrative distance than the primary route, so it stays out of the routing table until the primary disappears. For example, if OSPF (AD 110) learns the main path, ip route 0.0.0.0 0.0.0.0 203.0.113.1 200 installs only when OSPF loses the route. It is common for backup internet or 4G links.
33
Logs show MAC address flapping for several hosts between an access port and an uplink. What do you check?
MAC flapping means the switch sees the same source MAC arriving on two ports, which usually points to a Layer 2 loop, a misconfigured server NIC team, or a virtual machine host bridging two uplinks. Check show mac address-table for the MAC, trace both ports, verify STP state and look for unmanaged switches or hypervisor teaming in active-active mode without a matching port channel. Fix the loop or teaming, and keep BPDU Guard on access ports.
34
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.
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.
LACP (802.3ad, now 802.1AX) is the open standard with modes active and passive; at least one side must be active. PAgP is proprietary with modes desirable and auto; at least one side must be desirable. Mode on forces the channel with no negotiation, which can cause loops if the other side is not configured. LACP is preferred because it works with servers, firewalls and other vendors.
37
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.
38
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
39
What are the first hop redundancy protocols and how does HSRP work?
FHRPs give hosts a virtual gateway IP shared by two or more routers. HSRP is proprietary: routers elect an active router by highest priority (default 100), and the standby takes over the virtual IP and MAC if hellos stop. Preempt lets a higher priority router reclaim the role. VRRP is the open standard, and GLBP adds load sharing across several routers.
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.
Frequently asked questions
What does a Network Engineer do?
A network engineer designs, implements and troubleshoots routed and switched networks across campuses, branches and data centers. Work includes OSPF and static routing design, STP and EtherChannel tuning, HSRP gateways, VLSM addressing plans, change windows and root cause analysis for major incidents. Engineers with two to five years of experience and a CCNA, often working towards CCNP, are expected to lead changes and guide support engineers.
Which skills are tested in a Network Engineer interview?
Expect questions on OSPF and static routing, STP and EtherChannel design, IP address planning, Change management, Root cause analysis. Most rounds also ask you to explain a project and solve one practical problem live.
How do I prepare for a Network Engineer interview?
Revise Routing and OSPF, STP and EtherChannel, IP Addressing and Subnetting, Switching and VLANs first using the topic sheets, practise the questions on this page aloud, then do one full mock interview to practise timing.