ip helper-address [dhcp-server-ip]
Configures a DHCP relay agent on an interface to forward DHCP broadcast requests to a specific DHCP server IP address.
ip helper-address [dhcp-server-ip]When to Use This Command
- When DHCP clients and servers are on different subnets, and you need to forward DHCP broadcasts across a router.
- In a multi-VLAN environment where each VLAN has a DHCP server on a different subnet.
- When using a centralized DHCP server for multiple remote subnets to avoid deploying a DHCP server per subnet.
- To forward DHCP requests to a failover or secondary DHCP server for redundancy.
Command Examples
Basic DHCP relay configuration on a VLAN interface
interface Vlan10
ip helper-address 192.168.1.10Router(config)# interface Vlan10 Router(config-if)# ip helper-address 192.168.1.10 Router(config-if)# end Router# show ip interface Vlan10 | include Helper Helper address is 192.168.1.10
The command configures the VLAN10 interface to forward DHCP broadcasts to the DHCP server at 192.168.1.10. The 'show ip interface' output confirms the helper address is set.
Multiple DHCP relay addresses for redundancy
interface GigabitEthernet0/1
ip helper-address 10.0.0.1
ip helper-address 10.0.0.2Router(config)# interface GigabitEthernet0/1 Router(config-if)# ip helper-address 10.0.0.1 Router(config-if)# ip helper-address 10.0.0.2 Router(config-if)# end Router# show ip interface GigabitEthernet0/1 | include Helper Helper address is 10.0.0.1 [2]
Two helper addresses are configured for redundancy. The 'show ip interface' output shows the first helper address and the count of additional helpers in brackets. The router forwards DHCP requests to both servers.
Understanding the Output
The 'show ip interface' command displays the configured helper address. The line 'Helper address is <IP> [count]' shows the first helper address and the number of additional helper addresses configured. If no helper address is set, the line will not appear or show 'Helper address is not set'. A missing helper address means DHCP broadcasts will not be forwarded, causing clients on different subnets to fail to obtain an IP address. Multiple helper addresses are shown with a count in brackets; the router forwards DHCP packets to all listed servers.
CCNA Exam Tips
Remember that 'ip helper-address' forwards UDP broadcasts for DHCP (ports 67/68) by default, but also forwards other UDP services like TFTP, DNS, and NetBIOS unless disabled with 'ip forward-protocol'.
The command is configured in interface configuration mode, not globally. It must be applied to the interface facing the DHCP clients.
CCNA exam may test that the helper address must be reachable from the router; otherwise, DHCP requests are dropped.
Be aware that 'ip helper-address' does not forward broadcasts for all UDP services; only specific ports are forwarded by default.
Common Mistakes
Applying the helper address on the wrong interface (e.g., the interface facing the DHCP server instead of the clients).
Forgetting to configure a default route or ensure reachability to the DHCP server, causing relay failures.
Not realizing that multiple helper addresses cause the router to forward DHCP requests to all servers, which can lead to multiple IP offers and potential conflicts.
Related Commands
debug ip dhcp server events
Use this command to monitor DHCP server events in real time, such as address assignments, renewals, and conflicts, for troubleshooting DHCP operation on a Cisco IOS router acting as a DHCP server.
show ip interface
Displays the status and configuration of all IP interfaces on a Cisco router, including IP address, protocol status, and interface statistics, used for verifying interface IP configuration and troubleshooting connectivity issues.
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