A network administrator is configuring a Layer 3 router to act as a DHCP relay agent for a newly established guest wireless subnet on VLAN 75 (). Client devices on VLAN 75 send broadcast DHCPDISCOVER requests, but none of them acquire an IP address from the centralized DHCP server located at . Upon reviewing the router's configuration, the administrator discovers that the `ip helper-address 10.100.1.5` command was mistakenly applied to the outbound WAN interface facing the core network rather than the subinterface corresponding to VLAN 75. Which of the following best describes the immediate operational failure caused by this misconfiguration?
- The router drops or ignores the Layer 2 broadcast DHCPDISCOVER packets on the VLAN 75 subinterface because no active relay agent is listening on that ingress interface.Answer
- BThe router rewrites the DHCP payload using TCP port 67 instead of UDP port 67, causing the central DHCP server to discard incoming requests.
- CThe router forwards the client's broadcast request as a unicast packet, but populates the source port with UDP 68 instead of UDP 67.
- DThe central DHCP server receives the relayed request but attempts to issue an IP address lease from the WAN interface's IP scope instead of the VLAN 75 scope.
Answer
The router drops or ignores the Layer 2 broadcast DHCPDISCOVER packets on the VLAN 75 subinterface because no active relay agent is listening on that ingress interface.
DHCPDISCOVER packets are broadcast messages that stop at Layer 3 boundaries. For a router to convert these broadcasts into unicast messages directed to a central DHCP server, the relay agent (`ip helper-address`) must be enabled on the specific ingress interface (VLAN 75 subinterface) receiving the broadcasts. Placing the command on an outbound WAN interface means the router will never intercept the client broadcasts originating on VLAN 75.
Step-by-Step Solution
Key Concept
DHCP Relay Agent Interface Placement Requirements
Estimated Time:1m 30s