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Difficulty: HardDHCP Architecture and Relay Services

A network engineer is troubleshooting IP address acquisition issues for client hosts residing on VLAN 50 (172.16.50.0/24172.16.50.0/24). The centralized DHCP server (10.10.10.510.10.10.5) is active and reachable via ping from the core router. Packet captures indicate that client DHCPDISCOVERDHCPDISCOVER broadcast packets are successfully arriving at the switch Virtual Interface (SVI 50), but the Layer 3 switch fails to generate and forward unicast DHCP messages to the remote DHCP server. Which of the following configuration issues on the Layer 3 switch explains why the DHCP relay agent is unable to forward the client requests?

  1. The switch interface SVI 50 lacks an assigned IPv4 address, preventing the relay agent from populating the gateway IP address (giaddrgiaddr) field required for forwarding.Answer
  2. B
    The ip helper-address command was incorrectly configured on the switch interface connected to the DHCP server instead of the client-facing SVI 50 interface.
  3. C
    The relay agent attempted to establish a TCP session on port 67 to forward the request, which was dropped by the DHCP server.
  4. D
    The relay agent attempted to intercept client traffic on UDP port 67 instead of listening on UDP port 68.

Answer

The switch interface SVI 50 lacks an assigned IPv4 address, preventing the relay agent from populating the gateway IP address (giaddrgiaddr) field required for forwarding.
For a DHCP relay agent to successfully forward a client's broadcast request as a unicast packet, the receiving Layer 3 interface (SVI 50) must have a valid IP address. The relay agent writes this IP address into the gateway IP address (giaddrgiaddr) field of the DHCP header. The remote DHCP server relies on the giaddrgiaddr value to determine which IP address scope to allocate from. If no IP address is assigned to the SVI, the packet cannot be populated with a valid giaddrgiaddr and is dropped by the relay agent.

Step-by-Step Solution

1
Analyze the role of the DHCP relay agent (IP Helper) on a Layer 3 interface.
Identified that when a Layer 3 interface intercepts a DHCP broadcast (DHCPDISCOVERDHCPDISCOVER), it encapsulates the request into a unicast IPv4 packet targeted at the configured DHCP server IP.
Broadcast frames cannot traverse Layer 3 subnet boundaries natively.
2
Examine how the central DHCP server identifies which scope/pool to assign.
The relay agent inserts its own interface IPv4 address into the Gateway IP Address (giaddrgiaddr) field of the DHCP message header.
Without a valid giaddrgiaddr, the centralized DHCP server has no way of knowing which subnet pool the client belongs to.
3
Correlate the failure symptom with interface configuration prerequisites.
If SVI 50 does not have an IP address assigned, the relay agent cannot populate giaddrgiaddr and will drop the forwarding operation entirely.
An unnumbered or unconfigured L3 interface lacks the local network IP identity needed for DHCP relay processing.

Key Concept

DHCP Relay Agent and GIADDR Population
Estimated Time:2m 0s
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Topics

DHCP Architecture and Relay Services

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