PlantUML DiagramDHCP Relay Across Subnets
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You've built a network with centralized DHCP. The DHCP server sits in the data center on one VLAN. Clients on ten different subnets need to reach it. But DHCP DISCOVER is a broadcast, and broadcasts don't cross router boundaries. How do clients on the other subnets get IPs?
DHCP relay. The router on each subnet listens for DHCP broadcasts, converts them to unicast, and forwards them to the configured DHCP server. The server sees the request as coming from the router's interface (via the giaddr field), picks the matching pool, and replies. The router relays the response back to the client.
The key field is `giaddr` (Gateway IP Address). The relaying router sets this to the IP of the interface that received the broadcast. The server uses giaddr to determine which pool to pull an address from — the pool whose `network` matches giaddr's subnet.
! On the router interface facing clientsR1(config)# interface gigabitEthernet0/0R1(config-if)# ip helper-address 10.10.99.50R1(config-if)# exit! If there are multiple DHCP servers for redundancyR1(config)# interface gigabitEthernet0/0R1(config-if)# ip helper-address 10.10.99.50R1(config-if)# ip helper-address 10.10.99.51R1(config-if)# exit
The `ip helper-address` command forwards more than just DHCP. It relays several UDP broadcasts by default: TFTP (69), DNS (53), NetBIOS (137/138), BOOTP (67/68), and TACACS (49). If you only want DHCP relayed, use `no ip forward-protocol udp <port>` to disable others.
This statistic is on the DHCP server itself. It shows 182 DISCOVERs received and 180 OFFERs sent — normal ratio (2 missing offers are likely from clients that didn't send a REQUEST). If DISCOVERs drop to zero, the relay isn't reaching the server.
You have two DHCP servers, one primary and one backup, both should serve clients in VLAN 10. How do you configure relay on the router?
Select the safest next action.
DHCP relay seems simple — one command — but it's the source of countless subtle problems. Wrong helper address, helper missing on a VLAN, ACLs blocking UDP 67/68, giaddr mismatch. When a single subnet has DHCP issues, checking the helper config is step one.
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