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Zorluk: OrtaTroubleshooting IP Addressing and DHCP Services

A network technician is deploying a new smart HVAC controller on VLAN 55 (172.16.55.0/24172.16.55.0/24). The corporate DHCP server is centralized on VLAN 10 (172.16.10.10/24172.16.10.10/24). Upon booting up the controller, running a network diagnostics command shows the following output:

IPv4 Address. . . . . . . . . . . : 169.254.108.42
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
DHCP Enabled. . . . . . . . . . . : Yes

Other existing hosts on VLAN 10 obtain their DHCP leases without issues. Which of the following is the MOST likely cause of the HVAC controller failing to acquire an IP address?

  1. A
    The HVAC controller was configured with an incorrect default gateway residing outside its subnet.
  2. The router interface servicing VLAN 55 lacks an IP helper address configured to forward broadcast DHCP requests.Cevap
  3. C
    The DHCP relay service is misconfigured on the central DHCP server instead of the local workstation OS.
  4. D
    The DHCP scope for VLAN 55 became exhausted because usable host calculations included the network and broadcast addresses.

Cevap

The router interface servicing VLAN 55 lacks an IP helper address configured to forward broadcast DHCP requests.
The client's self-assignment of an Automatic Private IP Address (169.254.108.42169.254.108.42) indicates that its DHCPDISCOVER broadcast was never answered. Since the host resides on VLAN 55 and the DHCP server resides on VLAN 10, the Layer 3 boundary router between these subnets drops the broadcast unless an IP helper address (DHCP Relay agent) is explicitly configured on the VLAN 55 interface to convert the broadcast to unicast.

Adım Adım Çözüm

1
Analyze the terminal output from the HVAC controller.
The host assigned itself an APIPA address of 169.254.108.42/16169.254.108.42/16, indicating it sent DHCPDISCOVER broadcasts but received no DHCPOFFER responses.
When a client configured for dynamic addressing receives no response from a DHCP server, automatic private IP addressing (APIPA) assigns an address in the 169.254.0.0/16169.254.0.0/16 range.
2
Evaluate the network topology context.
The client is on VLAN 55 (172.16.55.0/24172.16.55.0/24) while the DHCP server is located on VLAN 10 (172.16.10.10/24172.16.10.10/24).
Routers drop Layer 2 broadcast frames by default, preventing broadcast DHCPDISCOVER packets from traversing subnets unless relayed.
3
Determine the required resolution mechanism.
An IP helper address (`ip helper-address 172.16.10.10`) must be configured on the VLAN 55 Layer 3 interface.
The IP helper agent converts incoming broadcast DHCPDISCOVER messages into unicast requests sent directly to the centralized DHCP server on VLAN 10.

Anahtar Kavram

Troubleshooting DHCP Relay / IP Helper across Subnets and APIPA Diagnosis
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Troubleshooting IP Addressing and DHCP Services

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