4.6 DHCP Client and Relay Flashcards
Configure and verify DHCP client and relay. (26 cards)
Define:
Dynamic Host Configuration Protocol (DHCP) client
A device that receives IP settings from a DHCP server.
DHCP clients automatically request IP configurations, reducing manual setup.
What is DHCP relay?
A router feature that forwards DHCP requests to a server.
DHCP relay is used when clients and servers are on different subnets, forwarding client requests to a centralized DHCP server.
How does a DHCP client obtain an IP address?
By sending a DHCP Discover message.
The client searches for DHCP servers to request an IP address and other network settings.
What command configures an interface as a DHCP client?
ip address dhcp
This command configures the interface to obtain an IP dynamically.
Fill in the blank:
The command ip address dhcp is used to configure a device as a ______ ______.
DHCP client
This makes the device request an IP from a DHCP server rather than using static configuration.
What are two commands to verify DHCP client configuration?
- show ip interface brief
- show ip dhcp binding
These commands help verify whether an interface has successfully obtained an IP address through DHCP and the status of the lease.
Which command checks the IP address assigned to VLAN 1 via DHCP?
show interfaces vlan 1
This command shows the interface’s assigned IP, its status, and other relevant information, including if it obtained the IP via DHCP.
What happens if a DHCP client cannot reach a DHCP server?
It assigns itself an APIPA address (169.254.x.x).
This occurs when the client cannot obtain an IP address from a DHCP server, leading it to self-assign an address in the APIPA range.
Why would a router be configured as a DHCP client?
To receive an IP from an upstream ISP.
Routers often use DHCP to obtain an IP address for WAN interfaces, simplifying configuration and reducing administrative effort.
What command is used to renew a DHCP-assigned IP on a Cisco router?
ip dhcp renew
This command initiates the DHCP renewal process for the interface, allowing the router to request a new IP lease from the DHCP server.
What command enables DHCP relay on a router?
ip helper-address <server-IP></server-IP>
This command configures the router to forward DHCP requests from clients to a specific DHCP server located on a different subnet.
How does ip helper-address help DHCP clients?
It forwards broadcast DHCP requests to a DHCP server.
DHCP broadcasts do not traverse routers, so this command ensures that DHCP messages from clients on one subnet are forwarded to a server on another subnet.
What are two commands to verify DHCP relay configuration?
- show running-config | include helper-address
- show ip dhcp relay
These commands confirm that the DHCP relay function has been properly configured and whether the ‘helper-address’ command is active.
Why does a router need an IP address on an interface to act as a relay?
To forward DHCP packets correctly.
The router uses the IP address of its interface to ensure that DHCP messages are routed and can reach the appropriate DHCP server.
Fill in the blank:
The command **ip helper-address <server-IP>** is configured on the \_\_\_\_\_\_ interface.</server-IP>
client-facing
This ensures the router forwards DHCP requests from client subnets to the server.
What command checks the status of DHCP relay traffic on a router?
debug ip dhcp server packet
This command provides real-time logs for diagnosing problems with DHCP request forwarding and responses.
What could cause a DHCP client to fail to obtain an IP address?
Misconfigured relay or incorrect helper address.
If the relay agent or helper address is misconfigured, DHCP requests won’t be forwarded correctly, preventing clients from getting an IP.
Why is DHCP relay required in multi-subnet networks?
Routers block DHCP broadcasts, so relay is necessary to forward requests.
Without relay, clients in different subnets would be unable to communicate with a centralized DHCP server.
True or False:
The ip helper-address command only forwards DHCP packets.
False
In addition to DHCP, the ip helper-address forwards other types of UDP broadcasts such as TFTP, DNS, and NetBIOS.
Fill in the blank:
A DHCP relay agent forwards requests using the IP address of its ______ interface.
incoming
The relay uses the source IP address of the incoming request to forward it to the correct DHCP server.
What is the purpose of the show ip dhcp binding command?
It displays active DHCP leases on the router.
This command allows network administrators to check which IPs have been assigned to DHCP clients.
Why should a router not have multiple ip helper-address entries?
It could cause multiple DHCP servers to respond, leading to conflicts.
Multiple helper addresses may lead to unpredictable IP assignments, so only one helper address per subnet should be configured.
What are two signs of a DHCP relay misconfiguration?
- No IP addresses are assigned to clients.
- Relay logs show no forwarded requests.
These issues often stem from incorrect or missing relay configuration.
What command shows DHCP relay statistics on a router?
show ip dhcp relay statistics
This command provides information about how many DHCP requests have been relayed and successfully forwarded.
