A network administrator configures a new client subnet on VLAN 40 () serviced by a central Layer 3 core switch. The central DHCP server () resides on VLAN 10. Users on VLAN 40 report they cannot access internal servers or the internet. Executing `ipconfig /all` on a affected workstation returns the following output:
text
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
Link-local IPv4 Address . . . . . : 169.254.109.88
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
The administrator verifies that the DHCP server scope for is active, enabled, and currently has over 500 available leases. Which of the following represents the most likely root cause of this failure?
- The Layer 3 interface for VLAN 40 lacks a DHCP relay agent (IP helper address) configuration pointing to 10.10.1.50.Answer
- BThe client host is configured with an invalid default gateway address causing it to fall back to an automatic link-local assignment.
- CThe central DHCP server scope for 10.40.0.0/22 has exhausted all usable host IP addresses.
- DThe client has been assigned a static IP address residing on a different subnet than the default gateway.
Answer
The Layer 3 interface for VLAN 40 lacks a DHCP relay agent (IP helper address) configuration pointing to 10.10.1.50.
Because DHCP requests originate as Layer 2 broadcast traffic (destination MAC FF:FF:FF:FF:FF:FF), routers and Layer 3 switches will not forward these frames across subnets. Configuring a DHCP relay agent (IP helper address) on the interface serving VLAN 40 allows the switch to encapsulate the broadcast request as a unicast packet sent directly to the DHCP server at 10.10.1.50.
Step-by-Step Solution
Key Concept
DHCP Relay Agent (IP Helper Address) placement across Layer 3 boundaries