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Lab ips-02 - DHCP Relay (ip helper-address)

Lab ips-02 - DHCP Relay (ip helper-address) - PingLabz Labs article title card

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-address command 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 helper implicitly 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.

Download CCNA Base Topology

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