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DHCP Interview Questions and Answers (Dynamic Host Configuration Protocol)

January 17, 2016

What is DHCP?
Dynamic Host Configuration Protocol (DHCP) assigns IP addresses to hosts dynamically. It allows easier administration and works well in small as well as very large network environments. All types of hardware can be used as a DHCP server including a Cisco router.

What information a DHCP server can provide to a host?
DHCP server can provide following information -
IP address
Subnet mask
Default gateway
Domain Name Server
WINS information

How DHCP Works?
DHCP works on DORA Process (DISCOVER - OFFER - REQUEST - ACKNOWLEDGEMENT).

1.When a Client needs an IP configuration, it tries to locate a DHCP server by sending a broadcast called a DHCP DISCOVER. This message will have a Destination IP of 255.255.255.255 and Destination MAC of ff:ff:ff:ff:ff:ff.
[Source IP - 0.0.0.0 , Destination IP - 255.255.255.255, Source Mac - Mac address of Host, Destination Mac - FF:FF:FF:FF:FF:FF]
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2.On Receiving DHCP Discover, Server sends a DHCP OFFER message to the client. The DHCPOFFER is a proposed configuration that may include IP address, DNS server address, and lease time. This message will be unicast and have the destination mac address of DHCP client's mac address. The source mac address will be that of the DHCP server.
[S.Mac - Mac address of Server , D.Mac - Mac address of Host]
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3.If the Client finds the Offer agreeable, it sends DHCP REQUEST Message requesting those particular IP parameters. This message will be a Broadcast message.
[Source Mac - Mac address of Host, Destination Mac - FF:FF:FF:FF:FF:FF]
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4.The Server on receiving the DHCP REQUEST makes the configuration official by sending a unicast DHCP ACK acknowledgment.
[Source Mac - Mac address of Server, Destination Mac - Mac address of Host]

What is the reason for getting APIPA address?
With APIPA, DHCP clients can automatically self-configure an IP address and subnetmask when a DHCP server is not available. When DHCP client boots up, it first looks for a DHCP server in order to obtain an IP address and subnet mask.
A client uses the self-configured IP address until a DHCP server becomes available. The APIPA service also checks regularly for the presence of a DHCP server. If it detects a DHCP server on the network, APIPA stops and the DHCP server replaces the APIPA networking addresses with dynamically assigned addresses.

What is the range of APIPA address?
The IP address range is 169.254.0.1 through 169.254.255.254. The client also configures itself with a default Class B subnet mask of 255.255.0.0.

What is the purpose of relay agent?
A DHCP relay agent is any host that forwards DHCP packets between clients and servers if server is not on the same physical subnet. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet.
DHCP relay agent can be configured using the ip helper-address command.

What is DHCP decline message?
It is Sent by Client to server indicating network address is already in use (already assigned to another device).

What is DHCPNAK message?
If the Server is unable to satisfy the DHCPREQUEST message (The requested network address has been allocated) the Server Should sent DHCPNAK message to client. It can also be Sent if client's notion of network address is incorrect (Client has moved to new subnet) or client's lease expired.

 

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