network engineer interview questions preview

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Network Engineer Interview Questions and Answers

250+ Questions and answers explained

100 Objective Type questions included with detailed explanation

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Preface

This Ebook is ideal for network engineers planning to attend interviews on

networking. The questions in the ebook are focused on fundamentals, technology and

scenarios. Questions are provided with answers along with detailed explanation.


1. What is the destination mac-address of an ARP request frame

Broadcast

2. Is ARP used on a PPP link

No

3. Does ARP contain an IP header

ARP is a layer 2 protocol. It does not use IP header.

4. What does a switch do when it receives an ARP request frame on one of its ports

It adds the source port to its mac-address table and broadcasts the frame out to all

ports.

5. Would an ARP request frame generated on a port which is member of VLAN 2,

be received on a port which is a member of VLAN 3

ARP is a layer 2 broadcast. VLAN’s are used for segregating broadcast domains. So

it would not be received.

6. Can an ARP packet cross IP networks

ARP is a layer 2 broadcast. It cannot traverse across IP networks.

7. If an ARP entry is available on the local cache of a computer, would an ARP

request be triggered

No. The computer would first check its ARP entry before sending out an ARP request

frame.

8. Explain how an ARP packet is triggered on a computer

Assume that a user on a computer pings another computer on the same network. For

the ping packet to be sent to the other computer, it has to be encapsulated on an

Ethernet frame. For constructing the frame, the mac-address of the destination is


needed. The computer checks its ARP cache if the corresponding ARP entry for the

destination computer is available. If not an ARP request is sent out.

9. Does a hub support ARP

If the hub works on mediums like Token Ring, Ethernet etc, then ARP would be

supported.

10. 3 switches SW1,SW2 and SW3 are interconnected and STP deployed on the

switches. PC1 is on SW1 PC2 is on SW2 and PC3 is connected to SW3. All the

PC's are on the same network address of 192.168.1.0/24. PC1 triggers ARP to

find the mac-address of PC3. Will the ARP packet reach the PC3

All the PC's are on the same network address. The PC's are on 3 switches which

makes it one large Layer 2 broadcast domain. So an ARP request which is a layer 2

broadcast will be received on all the ports on all the switches and correspondingly the

PC's

11. Thomas spoofs the mac-address of his PC to that of George. He then attempts an

ARP request on the LAN network to find the mac-address of the gateway. What

will be the sender mac-address value in the ARP protocol header

The sender mac-address inside the ARP protocol header would be the mac-address of

the device which initiates the ARP request. In this case, Thomas has edited his mac-

address and the current value is that of George. This would cause the sender mac-

address inside the ARP protocol header to be reflected to that of George.

12. In a DHCP environment, which of the components (Client or Server) initiates

the request

The client would always initiate the DHCP request. The request is triggered from the

client, when the TCP/IP adapter is configured for the option “Obtain IP address

automatically”. When this is selected, the operating system would automatically

generate the DHCP Discover packet to identify valid DHCP Servers on the network.

13. How many DHCP packets are exchanged between a client and a server before

the client receives an IP address

4 DHCP packets are exchanged between the client and the server. They are DHCP

Discover, DHCP Offer, DHCP Request, DHCP Ack.


14. What type of packet is a DHCP Discover packet

DHCP Discover is a layer 3 broadcast packet with destination IP address,

255.255.255.255

15. What is an IP Helper address feature and why is it required in a DHCP

environment

DHCP Discover packets are broadcast packets. This means that a DHCP Discover

packet which is sent from the client would not reach the DHCP Server, if the server

resides on a different network. This is because, routers are required for

communication between different networks and routers do not forward broadcast

packets. The IP Helper address feature is configured on the router. The feature

informs the router the DHCP Servers IP address for the network. So, when the router

receives the DHCP Discover packet, it would convert it from broadcast to unicast

packet and then send it to the DHCP Server.

16. What is a DHCP Scope and why is it required

A scope includes information like IP address ranges, Subnet Masks, gateway address,

DNS Server etc. This required so that clients would require the necessary information

from the server.

17. What would happen if there are multiple DHCP servers on a network

The clients would trigger DHCP Discover packets and the server which responds first

would provide the IP address to the client

18. How does a client know that a lease has expired and how is it renewed

When a client receives an IP address from the DHCP Server, the lease expiry date and

time is provided as part of DHCP options. This is maintained and recorded by the client.

Just before the lease expiry time, the client would initiate a renew request to the server

for a new lease

19. How many DHCP Servers can reside on one subnet

It’s unlimited. But if there is a DHCP client, which server would lease out the IP address

? It’s a simple logic. The first server which responds to the DHCP clients IP address

request would lease out the IP address.

network engineer interview questions preview

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