7.4.4 Module Quiz - DHCPv4 (Answers)

7.4.4 Module Quiz – DHCPv4 Answers

1. A DHCP-enabled client PC has just booted. During which two steps will the client PC use broadcast messages when communicating with a DHCP server? (Choose two.)

DHCPNAK
DHCPREQUEST
DHCPOFFER
DHCPACK
DHCPDISCOVER

Explanation: All DHCP messages between a DHCP-enabled client and a DHCP server are using broadcast messages until after the DHCPACK message. The DHCPDISCOVER and DHCPREQUEST messages are the only messages that are sent by a DHCP-enabled client. All DHCP messages between a DHCP-enabled client and a DHCP server use broadcast messages when the client is obtaining a lease for the first time.

2. An administrator issues the commands:

Router(config)# interface g0/1
Router(config-if)# ip address dhcp

What is the administrator trying to achieve?

configuring the router to act as a DHCPv4 server
configuring the router to act as a relay agent
configuring the router to resolve IP address conflicts
configuring the router to obtain IP parameters from a DHCPv4 server

Explanation: The ip address dhcp command activates the DHCPv4 client on a given interface. By doing this, the router will obtain the IP parameters from a DHCPv4 server.

3. When a client is requesting an initial address lease from a DHCP server, why is the DHCPREQUEST message sent as a broadcast?

The client does not yet know the IP address of the DHCP server that sent the offer.
The client may have received offers from multiple servers, and the broadcast serves to implicitly decline those other offers.
The client does not have a MAC address assigned yet, so it cannot send a unicast message at Layer 2.
The DHCP server may be on a different subnet, so the request must be sent as a broadcast.

Explanation: During the initial DHCP exchange between a client and server, the client broadcasts a DHCPDISCOVER message looking for DHCP servers. Multiple servers may be configured to respond to this request with DHCPOFFER messages. The client will choose the lease from one of the servers by sending a DHCPREQUEST message. It sends this message as a broadcast so that the other DHCP servers that sent offers will know that their offers were declined and the corresponding address can go back into the pool.

4. Which DHCP IPv4 message contains the following information?

Destination address: 255.255.255.255
Client IPv4 address: 0.0.0.0
Default gateway address: 0.0.0.0
Subnet mask: 0.0.0.0

DHCPDISCOVER
DHCPOFFER
DHCPACK
DHCPREQUEST

Explanation: A client will first send the DHCPDISCOVER broadcast message to find DHCPv4 servers on the network. This message will have the limited broadcast address, 255.255.255.255, as the destination address. The client IPv4 address, the default gateway address, and subnet fields will all be 0.0.0.0 because these have not yet been configured on the client. When the DHCPv4 server receives a DHCPDISCOVER message, it reserves an available IPv4 address to lease to the client and sends the unicast DHCPOFFER message to the requesting client. When the client receives the DHCPOFFER from the server, it sends back a DHCPREQUEST broadcast message. On receiving the DHCPREQUEST message, the server replies with a unicast DHCPACK message.

5. What kind of message is sent by a DHCPv4 client requesting an IP address?

DHCPDISCOVER unicast message
DHCPDISCOVER broadcast message
DHCPOFFER unicast message
DHCPACK unicast message

Explanation: When the DHCPv4 client requests an IP address, it sends a DHCPDISCOVER broadcast message seeking a DHCPv4 server on the network.

6. As a DHCPv4 client lease is about to expire, what is the message that the client sends the DHCP server?

DHCPDISCOVER
DHCPREQUEST
DHCPACK
DHCPOFFER

Explanation: When a DHCP client lease is about to expire, the client sends a DHCPREQUEST message to the DHCPv4 server that originally provided the IPv4 address. This allows the client to request that the lease be extended.

7. What is the destination IP address when an IPv4 host sends a DHCPDISCOVER message?

192.168.1.1
255.255.255.255
0.0.0.0
224.0.0.1

Explanation: Because a DHCP client does not have a valid IPv4 address, it must use a broadcast IP address of 255.255.255.255 as the destination address to communicate with the DHCP server. The DHCPDISCOVER message sent by the client is the first message sent in order to make initial contact with a DHCP server.

8. If more than one DHCP server is available on the local network, in which order will DHCP messages be sent between a host and a DHCP server?

acknowledgment, request, offer, discover
request, discover, offer, acknowledgment
discover, offer, request, acknowledgment
request, acknowledgment, discover, offer

Explanation: A DHCP host broadcasts a DHCP discover message to locate available servers. If more than one DHCP server is available, each server will respond to the host with a unicast DHCP offer message, which offers a lease to the client. The client then broadcasts a DHCP request message that identifies the specific server and offer that the client will accept. The identified server will unicast a DHCP acknowledgment message to finalize the offer.

9. What is the most likely scenario in which the WAN interface of a router would be configured as a DHCP client to be assigned a dynamic IP address from an ISP?

There is a web server for public access on the LAN that is attached to the router.
The router is configured as a DHCP server.
The router is also the gateway for a LAN.
It is a SOHO or home broadband router.

Explanation: SOHO and home broadband routers are typically set to acquire an IPv4 address automatically from the ISP. The IP address that is assigned is typically a dynamic address to reduce the cost, but a static IP address is possible with more cost. However, if the router is assigned a dynamic IP address, DNS issues will result in the web server behind the router not being easily accessible to the public. Routers are typically also gateways for LANs, but this has no bearing on whether the router is configured as a DHCP client on its WAN link or not. Likewise, a router can be configured to be a DHCP client in order to obtain an IP address from the ISP, but at the same time, it can be configured as a DHCP server to serve the IP addressing for the devices on its LAN.

10. Which is a DHCPv4 address allocation method that assigns IPv4 addresses for a limited lease period?

manual allocation
dynamic allocation
pre-allocation
automatic allocation

Explanation: Dynamic allocation is the most commonly implemented allocation mechanism. It leases the IP parameters for a predefined period of time.

11. What is the reason why the DHCPREQUEST message is sent as a broadcast during the DHCPv4 process?

for routers to fill their routing tables with this new information
to notify other hosts not to request the same IP address
for hosts on other subnets to receive the information
to notify other DHCP servers on the subnet that the IP address was leased

Explanation: The DHCPREQUEST message is broadcast to inform other DHCP servers that an IP address has been leased.

12. How is a DHCPDISCOVER transmitted on a network to reach a DHCP server?

A DHCPDISCOVER message is sent with the IP address of the default gateway as the destination address.
A DHCPDISCOVER message is sent with a multicast IP address that all DHCP servers listen to as the destination address.
A DHCPDISCOVER message is sent with the broadcast IP address as the destination address.
A DHCPDISCOVER message is sent with the IP address of the DHCP server as the destination address.

Explanation: The DHCPDISCOVER message is sent by a DHCPv4 client and targets a broadcast IP along with the destination port 67. The DHCPv4 server or servers respond to the DHCPv4 clients by targeting port 68.

13. Which destination IPv4 address does a DHCPv4 client use to send the initial DHCP Discover packet when the client is looking for a DHCP server?

224.0.0.1
255.255.255.255
127.0.0.1
the IP address of the default gateway

Explanation: Broadcast communications on a network may be directed or limited. A directed broadcast is sent to all hosts on a specific network. A limited broadcast is sent to 255.255.255.255. When a DHCP client needs to send a DHCP Discover packet in order to seek DHCP servers, the client will use this IP address of 255.255.255.255 as the destination in the IP header because it has no knowledge of the IP addresses of DHCP servers.

14. Under which two circumstances would a router usually be configured as a DHCPv4 client? (Choose two.)

This is an ISP requirement.
The administrator needs the router to act as a relay agent.
The router is meant to provide IP addresses to the hosts.
The router is intended to be used as a SOHO gateway.
The router has a fixed IP address.

Explanation: SOHO routers are frequently required by the ISP to be configured as DHCPv4 clients in order to be connected to the provider.

15. Which address does a DHCPv4 server target when sending a DHCPOFFER message to a client that makes an address request?

broadcast MAC address
client hardware address
gateway IP address
client IP address

Explanation: When a DHCPv4 client does not have an IPv4 address, a DHCPv4 server will send a DHCPOFFER message back to the client hardware address of the requesting DHCPv4 client.

16. Which DHCPv4 message will a client send to accept an IPv4 address that is offered by a DHCP server?

Broadcast DHCPACK
Broadcast DHCPREQUEST
Unicast DHCPACK
Unicast DHCPREQUEST

Explanation: When a DHCP client receives DHCPOFFER messages, it will send a broadcast DHCPREQUEST message for two purposes. First, it indicates to the offering DHCP server that it would like to accept the offer and bind the IPv4 address. Second, it notifies any other responding DHCP servers that their offers are declined.

17. What is an advantage of configuring a Cisco router as a relay agent?

It can forward both broadcast and multicast messages on behalf of clients.
It can provide relay services for multiple UDP services.
It reduces the response time from a DHCP server.
It will allow DHCPDISCOVER messages to pass without alteration

Explanation: By default, the ip helper-address command forwards the following eight UDP services:
Port 37: Time
Port 49: TACACS
Port 53: DNS
Port 67: DHCP/BOOTP client
Port 68: DHCP/BOOTP server
Port 69: TFTP
Port 137: NetBIOS name service
Port 138: NetBIOS datagram service

18. A host on the 10.10.100.0/24 LAN is not being assigned an IPv4 address by an enterprise DHCP server with the address 10.10.200.10/24. What is the best way for the network engineer to resolve this problem?

Issue the default-router 10.10.200.10 command at the DHCP configuration prompt on the 10.10.100.0/24 LAN gateway router.
Issue the ip helper-address 10.10.100.0 command on the router interface that is the 10.10.200.0/24 gateway.
Issue the ip helper-address 10.10.200.10 command on the router interface that is the 10.10.100.0/24 gateway.
Issue the network 10.10.200.0 255.255.255.0 command at the DHCP configuration prompt on the 10.10.100.0/24 LAN gateway router.

Explanation: The DHCP server is not on the same network as the hosts, so DHCP relay agent is required. This is achieved by issuing the ip helper-address command on the interface of the router that contains the DHCPv4 clients, in order to direct DHCP messages to the DHCPv4 server IPv4 address.

19. What is accomplished by the ip dhcp excluded-address 10.10.4.1 10.10.4.5 command?

The DHCP server will ignore all traffic from clients with IPv4 addresses 10.10.4.1 to 10.10.4.5.
The DHCP server will not issue IPv4 addresses ranging from 10.10.4.1 to 10.10.4.5.
Traffic destined for 10.10.4.1 to 10.10.4.5 will be denied.
Traffic from clients with IPv4 addresses 10.10.4.1 to 10.10.4.5 will be denied.

Explanation: The router functioning as the DHCPv4 server assigns all IPv4 addresses in a DHCPv4 address pool except addresses specified by the ip dhcp excluded-address low-address [high-address] global config command.

20. Which Windows command combination would enable a DHCPv4 client to reinstate its IPv4 configuration?

Enter ip config /release and then ip config /autonegotiate
Enter ip config /release and then ip config /renew
Enter ipconfig /release and then ipconfig /autonegotiate
Enter ipconfig /release and then ipconfig /renew

Explanation: The ipconfig /release Windows command releases the current host IPv4 configuration and the ipconfig /renew Windows command attempts to renew the IPv4 addressing with the DHCPv4 server.

21. Which command issued on R1 can be used to verify the current IPv4 address and MAC address binding?

R1# show ip dhcp binding
R1# show ip dhcp pool
R1# show ip dhcp server statistics
R1# show running-config | section dhcp

Explanation: The show ip dhcp binding command will show the leases, including IPv4 addresses, MAC addresses, lease expiration, type of lease, client ID, and username.

22. Which DHCP operation statement is true?

A DHCP client must wait for lease expiration before sending a new DHCPREQUEST message.
If a DHCP client receives several DHCPOFFER messages from different servers, it sends a unicast DHCPACK message to the selected server.
The DHCPDISCOVER message contains the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway.
When a DHCP client boots, it broadcasts a DHCPDISCOVER message to identify an available DHCP server on the network.

Explanation: The client broadcasts a DHCPDISCOVER message to identify any available DHCP servers on the network. A DHCP server replies with a DHCPOFFER message. This message offers to the client a lease that contains such information as the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway. After the client receives the lease, the received information must be renewed through another DHCPREQUEST message prior to the lease expiration.