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DHCP Questions

February 6th, 2017 Go to comments

Question 1

Explanation

The command “show ip dhcp conflict” is used to display address conflicts found by a Cisco IOS DHCP Server when addresses are offered to the client. An example of the output of this command is shown below:

show_ip_dhcp_conflict.jpg

Question 2

Explanation

We can use the “lease” command to specify the duration of the lease: lease {days [hours][minutes] | infinite}

The default is a one-day lease.

Question 3

Explanation

An address conflict occurs when two hosts use the same IP address. During address assignment, DHCP checks for conflicts using ping and gratuitous ARP. If a conflict is detected, the address is removed from the pool. The address will not be assigned until the administrator resolves the conflict.

(Reference: http://www.cisco.com/en/US/docs/ios/12_1/iproute/configuration/guide/1cddhcp.html)

Question 4

Explanation

The following example shows how to configure a DHCP Server on a Cisco router:

Configuration Description
Router(config)#ip dhcp pool CLIENTS Create a DHCP Pool named CLIENTS
Router(dhcp-config)#network 10.1.1.0 /24 Specifies the subnet and mask of the DHCP address pool
Router(dhcp-config)#default-router 10.1.1.1 Set the default gateway of the DHCP Clients
Router(dhcp-config)#dns-server 10.1.1.1 Configure a Domain Name Server (DNS)
Router(dhcp-config)#domain-name 9tut.com Configure a domain-name
Router(dhcp-config)#lease 0 12 Duration of the lease (the time during which a client computer can use an assigned IP address). The syntax is “lease {days[hours] [minutes] | infinite}”. In this case the lease is 12 hours. The default is a one-day lease.
Before the lease expires, the client typically needs to renew its address lease assignment with the server
Router(dhcp-config)#exit
Router(config)# ip dhcp excluded-address 10.1.1.1 10.1.1.10 The IP range that a DHCP Server should not assign to DHCP Clients. Notice this command is configured under global configuration mode

Note: We checked with both Cisco IOS v12.4 and v15.4 but found no “ip dhcp-server pool” command:

ip_dhcp.jpg

Therefore the answer “ip dhcp-server pool …” is not correct.

Question 5

Explanation

Quick review of DHCP Spoofing and DHCP snooping:

DHCP_Spoofing_Attack.jpg

DHCP spoofing is a type of attack in that the attacker listens for DHCP Requests from clients and answers them with fake DHCP Response before the authorized DHCP Response comes to the clients. The fake DHCP Response often gives its IP address as the client default gateway -> all the traffic sent from the client will go through the attacker computer, the attacker becomes a “man-in-the-middle”.

The attacker can have some ways to make sure its fake DHCP Response arrives first. In fact, if the attacker is “closer” than the DHCP Server then he doesn’t need to do anything. Or he can DoS the DHCP Server so that it can’t send the DHCP Response.

DHCP snooping can prevent DHCP spoofing attacks. DHCP snooping is a Cisco Catalyst feature that determines which switch ports can respond to DHCP requests. Ports are identified as trusted and untrusted.

DHCP_Spoofing_Attack_Trust_Untrust_Ports.jpg

Only ports that connect to an authorized DHCP server are trusted, and allowed to send all types of DHCP messages. All other ports on the switch are untrusted and can send only DHCP requests. If a DHCP response is seen on an untrusted port, the port is shut down -> Answer D is correct.

The DHCP snooping binding database has the MAC address, the IP address, the lease time, the binding type, the VLAN number, and the interface information that corresponds to the local untrusted interfaces of a switch. By using this database, DHCP can bind a particular IP Address to a particular client via its unique MAC address. This is called DHCP Reservation -> B is correct.

Comments (2) Comments
  1. anonymous
    February 19th, 2017

    Question 5 are you sure that the answer is not C,D instead of B,D?

    as many other websites saying
    https://quizlet.com/180803168/ccna-rahim-flash-cards/

  2. punisher
    February 21st, 2017

    C is definitely correct. Don’t understand B…