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
C. DHCP relay protocol

Correct Answer: A

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.