CISCO CCNA 200-301 Q86

Which feature enables a host to obtain an IP address from a DHCP server on another subnet?

A. DHCP relay agent
B. DHCP BOOTP agent
C. DHCP relay protocol
D. DHCP BOOTP relay

Correct Answer: A

Explanation:
A Dynamic Host Configuration Protocol (DHCP) relay agent enables hosts to obtain IP addresses from a DHCP server on another subnet. Hosts use DHCPDISCOVER broadcast messages to locate the DHCP server because they don’t know the location of the DHCP server. Because routers are designed to filter broadcasts, the DHCPDISCOVER packet would be dropped unless the router is configured to forward such packets. Enabling a DHCP relay agent on a Cisco router allows it to receive certain types of broadcasts and forward them to special helper addresses.

The following sequence describes an IP address relay process:

– The DHCP client broadcasts a DHCP request on the network.
– The DHCP request is intercepted by the DHCP relay agent, which inserts the relay agent information option (option 82) in the packet.
– The DHCP relay agent forwards the DHCP packet to the DHCP server.
– The DHCP server uses the suboptions of option 82 in the packet, assigns IP addresses and other configuration parameters, and forwards the packet to the client.
– The relay agent again intercepts the packet and strips off the option 82 information before sending it to the client.

The ip helper-address interface configuration command enables a DHCP relay agent on a Cisco router.

DHCP is an enhancement over Bootstrap Protocol (BOOTP) and is used to automate the distribution of IP address to clients from a central server. The BOOTP protocol was also used distribute IP addresses, but was inflexible to changes in the network. DHCP offers three advantages that also address the inflexibility of the BOOTP protocol:

– Automatic allocation of permanent IP addresses
– Automatic allocation of time bound (leased) IP addresses
– Ability to assign static IP address or define a pool of reserved IP address

When a DHCP relay is unnecessary, the following steps describe the address allocation process:

– The client device broadcasts a DHCPDISCOVER broadcast message to locate a DHCP server.
– The DHCP server replies with a DHCPOFFER unicast message containing configuration parameters, such as an IP address, a MAC address, a domain name, and a lease for the IP address for the client device.
– The client sends back a DHCPREQUEST broadcast, which is a formal request for the offered IP address to the DHCP server.
– The DHCP server replies back to client device with DHCPACK unicast message, acknowledging the allocation of the IP address to this client device.

While DHCP is very useful in reducing the administrative burden of issuing IP configurations in a large network, Cisco best practices call for using static IP addressing in a small (6 or fewer hosts) network.

All other options are invalid devices or features.

CISCO CCNA Exam – Q247

Which statement describes the process of dynamically assigning IP addresses by the DHCP server?

A. Addresses are allocated after a negotiation between the server and the host to determine the length of the agreement.
B. Addresses are permanently assigned so that the hosts uses the same address at all times.
C. Addresses are assigned for a fixed period of time, at the end of the period, a new request for an address must be made.
D. Addresses are leased to hosts, which periodically contact the DHCP server to renew the lease.

Correct Answer: D

Explanation:
The DHCP lifecycle consists of the following:
Allocation: A client begins with no active lease, and hence, no DHCP-assigned address. It acquires a lease through a process of allocation.

Reallocation: If a client already has an address from an existing lease, then when it reboots or starts up after being shut down, it will contact the DHCP server that granted it the lease to confirm the lease and acquire operating parameters. This is sometimes called reallocation; it is similar to the full allocation process but shorter.

Normal Operation: Once a lease is active, the client functions normally, using its assigned IP address and other parameters during the “main part” of the lease. The client is said to be bound to the lease and the address.

Renewal: After a certain portion of the lease time has expired, the client will attempt to contact the server that initially granted the lease, to renew the lease so it can keep using its IP address.

Rebinding. If renewal with the original leasing server fails (because, for example, the server has been taken offline), then the client will try to rebind to any active DHCP server, trying to extend its current lease with any server that will allow it to do so.

Release: The client may decide at any time that it no longer wishes to use the IP address it was assigned, and may terminate the lease, releasing the IP address.

CISCO CCNA Exam – Q243

How does a DHCP server dynamically assign IP addresses to hosts?

A. Addresses are permanently assigned so that the host uses the same address at all times.
B. Addresses are assigned for a fixed period of time. At the end of the period, a new request for an address must be made, and another address is then assigned.
C. Addresses are leased to hosts. A host will usually keep the same address by periodically contacting the DHCP server to renew the lease.
D. Addresses are allocated after a negotiation between the server and the host to determine the length of the agreement.

Correct Answer: C

Explanation:
DHCP works in a client/server mode and operates like any other client/server relationship. When a PC connects to a DHCP server, the server assigns or leases an IP address to that PC. The PC connects to the network with that leased IP address until the lease expires. The host must contact the DHCP server periodically to extend the lease. This lease mechanism ensures that hosts that move or power off do not hold onto addresses that they do not need. The DHCP server returns these addresses to the address pool and reallocates them as necessary.

CISCO CCNA Exam – Q242

Refer to the exhibit.
CISCO CCNA Exam – Q242

Which rule does the DHCP server use when there is an IP address conflict?

A. The address is removed from the pool until the conflict is resolved.
B. The address remains in the pool until the conflict is resolved.
C. Only the IP detected by Gratuitous ARP is removed from the pool.
D. Only the IP detected by Ping is removed from the pool.
E. The IP will be shown, even after the conflict is resolved.

Correct Answer: A

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.

CISCO CCNA Exam – Q239

When a DHCP server is configured, which two IP addresses should never be assignable to hosts? (Choose two.)

A. network or subnetwork IP address
B. broadcast address on the network
C. IP address leased to the LAN
D. IP address used by the interfaces
E. manually assigned address to the clients
F. designated IP address to the DHCP server

Correct Answers: A, B

Explanation:
Network or subnetwork IP address (for example 11.0.0.0/8 or 13.1.0.0/16) and broadcast address (for example 23.2.1.255/24) should never be assignable to hosts. When try to assign these addresses to hosts, you will receive an error message saying that they can’t be assignable.