Interview Questions and Answers

Here are the sample answers for interview questions for Network Engineer/Administrator/Technician job position.  

Top Networking Interview Questions and Answers

Sample QuestionsSample Answers
Differentiate between hub and switch.Hub has single collision domain and single broadcast domain, while Switch can support single broadcast domain and multiple collision domains.
What is a router?Router is used to forward data packets to computer networks
What are the OSI Model Layers? Give brief description and examplesOSI or Open Systems Interconnection model consists of 7 Layers that describes how telecommunication or computing systems use to communicate or operate over a network. For more info, refer to this 7 Layers of OSI Model: Functions and Examples and Layers of OSI Model Mnemonics
3. What is TCP/IP Model?Give brief description and examplesThe TCP/IP model is the default method of data communication on the Internet. For more info, refer to this OSI vs TCP/IP Network Models Comparison
4. Differentiate between OSI Model and TCP/IP Model-OSImodel has 4 while TCP/IP model has 7 layers -OSI is horizontal approach while TCP/IP model is veritical approach For more additional comparison, refer to this -> OSI vs TCP/IP Network Models Comparison
Differentiate between Physical and Logical Topology.Physical topology is the actual place where the network elements are located and connected, while Logical Topology is how the traffic flows across the network.
What are different types of Network Topologies?Bus Topology – all network devices are connected to a common cable or bus Star Topology – network devices are connected to a central device/controller Ring Topology – network devices are connected to two or other devices, forming a single continuous path Mesh Topology – each device of the network is connected to all other devices of the network Tree Topology – combination of star and bus topology Hybrid topology – combination of different topologies
5. What is NAT?Briefly explain diff types.– Static NAT – is one to one mapping of an IP to another IP address -Dynamic NAT – mapping of an IP address to a group of IP addresses (NAT pool) -PAT (Port Address Translation) – is a dynamic NAT that can map multiple IP to a single IP
6.Differentiate between TCP & UDP.TCP is connetion-oriented communication protocol while UDP is connectionless. Refer to this for more info ->
7. What is TCP three-way handshake?TCP uses 3 way handshake to establish connection as follows: 1) SYN – client initiates connection by sending TCP SYN packet to destination server 2)SYN-ACK – server receives the packet and responds with SYNC-AC 3)ACK – client acknowledge the response from the server
8. What is IP address and types?Refer to this -> IPv4 Classful and Reserved Addresses
9. What is VLAN?VLAN is logical segmentation of a layer2 broadcast domain or L2 network. Somebenefits are logical grouping of users/functions, reduce broadcast storm by increasing number of broadcast domain and enchance network security.
10.What is Trunk?Trunk is a port that carry frames of multiple VLANs
What are VLAN Tagging Methods?Inter Switch Link (ISL) which is Cisco Proprietary and 802.1Q(Dot1Q) which is an IEEE standard.
11. What is STP?STP or Spanning Tree Protocol is an L2 link management protocol providing path redundancy while preventing loops in network. It uses BPDU to break loops and forces redundant path into a standby or blocked state.
12. What are STP Port states?Disabled -> Blocking -> Listening -> Learning -> Forwarding
What is Root and Designated Port?Root port = single selected port on switch, other than root switch, with least path cost to reach root bridge Designated port = port that has the lowest Spanning Tree Path Cost on a network segment Key notes: – a root port can never be a designated port – there can be only one root port on a switch, but it may have multiple designated ports -all ports on a root bridge(root switch) are designated port
What are types of STP-STP (Spanning Tree Protocol) -RSTP (Rapid Spanning Tree Protocol) -CST(Common Spanning Tree Protocol) -PVST( Per Vlan Spanning Tree Protocol) ) RPVST (Rapid Per Vlan Spanning Tree Protocol)
How to elect root bridge in STP?STP elects root bridge based on priority and mac address. The lower priority, the better. If priority is same, mac address is tie-breaker. Root bridge is selected by configuring manually its bridge priority to low value. Default value is 32768 (range is 0 to 61440).
What is default priority value of switch?32768
What is range of switch priority value?0 to 61440
How much value the priority is incremented?4096
What is network redundancy?Network Redundancy is process of adding additional instances of network devices and communication lines to help ensure network availability. Basically adding alternate network paths implemented through standby routers/switches.
What are protocols used in redundancy?-HSRP -multicast address is -VRRP -GLBP used for load balancing and high availability
What is virtual MAC address range for HSRPv2?0000.0c9f.fxxx
What are the load balancing methods for VRRP?-Round-Robin -Host-dependant -Weighted
What are the protocols used in Ethernet Channel?– PAGP (Port Aggregaton Protocol) -LACP (Link Aggregation Protocol)
What are Forms of Network Redundancy?– Fault Tolerance – providing full hardware redundancy and mirror application across two or more systems – High Availability is a software-based redundancy that uses clusters of servers
13. What are Routing Protocols?Interior Gateway Protocols (IGP) either Distance Vector (RIP,IGRP) or Link State(OSPF,IS-IS). Exterior Gateway Protocols (EGP) like BGP
14. What is AD?Give sample AD valuesAdministrative Distance refers to the reliability of the routing protocol. Lower AD value = Higher preference to routing protocol. Refer to this -> Cisco’s Administrative Distance Juniper’s Route Preference and Huawei’s Preference Cheat Sheet . AD values = Tie Breaker for Routing protocol selection)
15. What is Metric?Metric is used in calculation of paths by routing protocols. The path having the lower metric = best route and installed in the routing table. If there are multiple paths to reach same destination and having same metric, loadbalancing will take place. (Metric values = Tie Breaker for Best Route selection)
16. What is OSPF?Explain briefly.OSPF or Open Shortest Path First is a vendor independent link-state IGP routing protocol. Key things of OSPF uses multicast address to send hello to form neighbor relationship hello time is 10 secs, dead time interval is 30 secs -it sends only triggered update if there’s network change -it has full view of network topology in topology table
How OSPF chooses DR?-Highest Priority -Highest Router ID -Highest IP address
17. Discuss OSPF neighbor formation states.Init State -> Two-way State-> ExStart State->Exchange State->Loading State -> Full State
What is purpose of Area in OSPF?– it reduces overhead on routing table -less time is required to run SFP algorithm ( since routers need to recalculate link state DB when there’s topology change) -routing updates are reduced
What are the OSPF LSAs?-LSA 1 router LSA -LSA 2 network LSA -LSA 3 summary LSA -ASBR summary LSA -ASBR LSA -Reserved for Future Use -LSA 7 NSSA LSA
What are the condition for OSPF to form neighbor?-Area type -Area -Network -Authentication -Hello/Dead Interval
What is loop prevention mechanism in OSPF?OSPF has an Area 0 or Backbone area to avoid loop wherein every area must pass traffic from backbone area. OSPF also has Router ID used to identify the routers.
What is ABR and ASBR?ABR or Area Border Router is a router used to establish a connection between backbone area and other OSPF areas.It has at least one of its interface is connected to area 0. ASBR or Autonomous System Border Router is a router that is connected to other OSPF areas, as well as other routing protocols. It normally advertises routes from other routing systems into the OSPF area.
Why OSPF requires all traffic between non-backbone areas to pass to backbone area(area 0) 
Explain BGP and its Best Path Selection Algorithm.BGP or Border Gateway Protocol is a routing protocol for the internet. It is used to exchange routing and reachability information among autonomous systems(AS) on the internet. BGP best path algorithm decides the best path to install in the routing table and to use to forward traffic. For the path selection, refer to this Border Gateway Protocol (BGP) Best Path Selection Mnemonic
What is BGP Loop Prevention Mechanism?The rule is if you see your own AS number in the AS path, you don’t accept it
What is BGP’s default local preference?100
What is BGP’s default MED(metric) value?0
18. Discuss BGP split-horizon ruleWhen BGP router receives an update from another internal BGP router, it will not forward this information to another internal BGP router
Explain between Stateful and Stateless FirewallStateful firewall maintains the state of every connection going through the firewall. It keeps track through the use of source and destination address, port number and IP flags. It begin with 3-way handshake(SYN,SYN-ACK,ACK) and typically ends with 2-way exchange(FIN,ACK)
What is default route?Default route is an IP configuration that establishes forwarding rule of packets when no specific address of a next-hop is available from the routing table.
How to check connectivity between source and destination?You can use network tools like ping and traceroute
What is path selection criteria of router?-Longest Prefix Match -Minimum AD value -Best metric value
Explain briefly about Unicast, Multicast,Broadcast and Anycast)Unicast – only 1 receiver. (1 to 1 ) Multicast – one more receiver but only members of multicast group receives the traffic (1 to many) Broadcast – one more receiver but all receivers receive the traffic (1 to all) Anycast – is received by nearest receiver in a group of the receivers that has same IP
What is Broadcast address in Networking?IP address: or MAC address: ffff:ffff:ffff or ff:ff:ff:ff:ff:ff
What are the multicast addresses used in IPv4 and IPv6 by different routing protocols?OSPF (IPv4)=, OSPF(IPv6)=ff02::5/ff02::6 RIP (IPv4)= RIPng(IPv6)=ff02::9 EIGRP(IPv4)= EIGRP(IPv6)=ff02::10
What is APIPA?APIPA is Automatic Private IP Addressing. It uses the Class B ( to If Autoconfiguration enabled, then APIPA is set. It will auto assign if there’s no DHCP server and not using static IP.
What is SNMP and elements?SNMP or Simple Network Management Protocol is used for network traffic management. It uses port tcp/udp 161. Three elements are: SNMP manager (network management apps) SNMP agents (installed in a managed device) MIB (Management Information Base) or database object that agent can use to populate data.
What are major functions of SNMPv3?-Authentication -Encryption Message Integry ( uses HMAC-SHA for security)