Lab 15: DHCP Relay Across Subnets
Real networks don't run a DHCP server per subnet — they use one central server + relays. Configure ip helper-address so clients two hops away still lease addresses. Difficulty: Intermediate · Time: ~30 min.
Lab objectives
- Run a central DHCP server for a REMOTE subnet's pool
- Configure ip helper-address on the client-side gateway
- Understand broadcast→unicast relay
- Verify leases land across the router
Topology & addressing
R1 connects client LAN 192.168.20.0/24 (Gi0/0 = .1) and server LAN 192.168.99.0/24 (Gi0/1 = .1). DHCP server at 192.168.99.5 with a pool FOR 192.168.20.0/24 (gateway .1). PCs on the client LAN set to DHCP.
Step-by-step configuration
Server: pool for 192.168.20.0/24, default-router 192.168.20.1 | The central server holds the REMOTE subnet's pool |
R1: interface gi0/0ip helper-address 192.168.99.5 | Relay client broadcasts as unicast to the server |
| PCs → DHCP | Watch them lease 192.168.20.x from across the router |
Verification
PCs receive 192.168.20.x addresses even though the server sits on another subnet — remove the helper-address and leases stop (requests die at the router, since broadcasts don't cross it). That before/after is the whole lesson.
Next lab: labs hub · test yourself: CCNA practice test.
Frequently asked questions
Why doesn't DHCP work across routers by default?
DHCP Discover is a broadcast, and routers don't forward broadcasts — the request never reaches a remote server.
What exactly does ip helper-address do?
The router converts the client's broadcast into a unicast to the specified server, inserting the client subnet so the server picks the right pool.
Where is the helper address configured?
On the CLIENT-facing interface of the gateway router — where the broadcasts arrive.
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