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Difficulty: MediumDHCP Architecture and Relay Services

Clients connected to VLAN 20 (192.168.20.0/24192.168.20.0/24) are unexpectedly receiving IPv4 address leases from the pool configured for VLAN 10 (192.168.10.0/24192.168.10.0/24). Inspection of the central DHCP server shows healthy scope configurations. Which of the following is the most likely cause of this issue?

  1. The relay agent command was configured on the Switch Virtual Interface (SVI) for VLAN 10 instead of VLAN 20, causing the gateway IP address (giaddrgiaddr) field to be populated with VLAN 10's interface IP.Answer
  2. B
    The DHCP relay agent converted the unicast DHCPDISCOVERDHCPDISCOVER broadcast into Layer 2 multicast frames, overriding the subnet scope selection mechanism.
  3. C
    The relay agent modified the source UDP port from 67 to 68 before forwarding the unicast packet to the central server.
  4. D
    The workstations failed to broadcast a DHCPDISCOVERDHCPDISCOVER packet and automatically generated an APIPA address within the VLAN 10 range.

Answer

The relay agent command was configured on the Switch Virtual Interface (SVI) for VLAN 10 instead of VLAN 20, causing the gateway IP address (giaddrgiaddr) field to be populated with VLAN 10's interface IP.
When a Layer 3 device acts as a DHCP relay agent, it captures client Layer 2 broadcast requests on a specific interface, populates the gateway IP address (giaddrgiaddr) field with that receiving interface's IP address, and unicasts the packet to the DHCP server. If the relay directive is placed on VLAN 10's interface instead of VLAN 20's, the giaddrgiaddr sent to the server will belong to VLAN 10, causing the DHCP server to select and lease an IP address from the VLAN 10 scope.

Step-by-Step Solution

1
Analyze how a DHCP relay agent identifies which scope to request from the central DHCP server.
The relay agent sets the Gateway IP Address (giaddrgiaddr) field in the DHCP message header to the IP address of the local interface (SVI/subinterface) that received the client's Layer 2 broadcast.
The central DHCP server inspects the giaddrgiaddr field to select the correct IP scope corresponding to that subnet.
2
Evaluate the symptom against the relay mechanism.
Clients on VLAN 20 receiving VLAN 10 leases indicates that the DHCP server received a request with a giaddrgiaddr belonging to VLAN 10.
This occurs when the helper/relay command is mistakenly applied to VLAN 10's interface instead of VLAN 20's interface.

Key Concept

DHCP Relay Gateway IP Address (giaddr) Population
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Topics

DHCP Architecture and Relay Services

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