Cisco IP Helper Address Configuration

Cisco IP Helper Address: Viewing and Changing DHCP Relay

IP helper addresses on a Cisco switch tell a VLAN where to forward DHCP broadcast traffic — essential when your DHCP server sits on a different subnet/VLAN than the clients requesting an address. Viewing the Current Configuration show ip int vlan # Replace # with your VLAN number. This returns the VLAN’s interface details, including […]

By Andrew Armstrong • 2 min read

IP helper addresses on a Cisco switch tell a VLAN where to forward DHCP broadcast traffic — essential when your DHCP server sits on a different subnet/VLAN than the clients requesting an address.

Viewing the Current Configuration

show ip int vlan #

Replace # with your VLAN number. This returns the VLAN’s interface details, including any IP helper address currently configured.

Changing the Helper Address

The example below assumes your current helper is 192.168.0.1 and you want to change it to 192.168.0.2:

config t
int vlan #
no ip helper-address 192.168.0.1
ip helper-address 192.168.0.2
exit
wr mem

wr mem saves the running configuration to the switch’s startup configuration — without it, the change will be lost on the next reboot. You can configure multiple helper addresses on the same VLAN interface if DHCP requests need to be relayed to more than one server.

IP Helper Forwards More Than Just DHCP

Worth knowing before you assume ip helper-address only matters for DHCP relay: by default a Cisco IP helper address relays eight specific UDP broadcast protocols to the target address, not just DHCP/BOOTP — including TFTP, DNS, NetBIOS Name Server and Datagram Server, TACACS, and Time Service. That’s usually harmless and often exactly what you want (PXE boot environments commonly rely on the same helper relaying both DHCP and TFTP traffic to the same server), but if you specifically want to relay DHCP only, you can scope it down with the ip forward-protocol udp global commands rather than relaying the full default set to every configured helper address.

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Andrew Armstrong

Andrew Armstrong is a UK-based IT professional with 26+ years of hands-on experience in Windows, Windows Server, SCCM/ConfigMgr, Active Directory, PowerShell, and enterprise infrastructure.

He founded TechyGeeksHome in 2010 and has published over 1,500 practical guides covering real-world IT problems and solutions. When not solving IT problems,

Andrew develops free Windows utilities including Ultimate Settings Panel, which has been downloaded over 850,000 times.

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