DHCP Relay: Missing Helper Address
DHCP clients broadcast, while centralized DHCP servers commonly live on another subnet. This lab builds the practical habit of checking the Layer 3 relay point before changing pools or client settings.
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.
show ip interface briefshow running-config interface vlan 20show ip dhcp server statisticsTopology 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?
No setup, no image sourcing. Book a session or ask for a live demo.