DHCP discover messages are L2 broadcasts - they do not cross routers. So when client and server are on different segments, you need DHCP relay (sometimes called "DHCP helper") to forward those broadcasts as unicasts to the server. The configuration is one command per interface. This lab configures R2 as the DHCP relay forwarding requests from its LAN side to R1 (the DHCP server from ips-01).
What you will learn
- The
ip helper-addresscommand and where to put it - How DHCP relay transforms L2 broadcasts into L3 unicasts to the server
- The GIADDR field and why it matters to the DHCP server
- What other broadcasts
ip helperimplicitly forwards (the default eight UDP ports)
What this lab does NOT cover
- Option 82 (relay agent information) - mostly relevant for service providers
- DHCPv6 relay
- Multiple helper-address destinations for redundancy (briefly mentioned)
Topology
Download the CCNA Base Topology .yaml
3 iol-xe routers + 1 alpine + 1 ioll2-xe managed switch + 1 unmanaged switch. The reusable foundation for the PingLabz CCNA Labs library.
