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CCNA · 200-301Intermediate

DHCP Relay: Missing Helper Address

One VLAN leases addresses fine and the next one never does. Diagnose the missing relay and fix it yourself, live, right below.

Interactive simIP ServicesDHCPRelay

Symptom

A remote VLAN broadcasts DHCPDISCOVER but the centralized server never sees it. Configure and verify relay forwarding with ip helper-address.

Your mission

Prove the cause, then fix it

Format

Guided CLI investigation

Start interactive labSee the investigation plan ↓Need a hint? Reveal the root cause ↓
DHCP Relay: Missing Helper Address

Investigate before you configure.

Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.

Observe the symptom and link state.

Diagnose by comparing the protocol evidence.

Verify the expected device state and confirm the original symptom is resolved.

Console access: R1

This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.

Need a hint?

Reveal the root cause when you're ready.

Try the investigation first, then use this as your escape hatch, not a dead end.

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Lab debrief

Turn the session into a repeatable troubleshooting pattern.

Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.

The problem

A client in VLAN 20 repeatedly sends DHCPDISCOVER. The DHCP server works for local clients but receives no requests from VLAN 20 because the gateway SVI is not relaying broadcasts to it.

What you'll practice

  • Explain why routers do not forward DHCP broadcasts by default
  • Identify the gateway interface for a client VLAN
  • Configure an IPv4 DHCP relay
  • Verify relay and server statistics
  • Confirm the client receives the correct lease

The topology

An access switch connects a VLAN 20 client to a multilayer gateway. A routed link reaches a centralized DHCP server on a different subnet.

Commands to run yourself

The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.

Check the client VLAN gateway
show ip interface brief
Inspect helper configuration
show running-config interface vlan 20
Check DHCP server counters
show ip dhcp server statistics

Topology diagram

Frequently asked

Why is ip helper-address configured on the gateway?

The gateway receives the client broadcast and converts it into a unicast relay message for the DHCP server on the remote subnet.

Ready to run this lab yourself?

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