Configuring a DHCP Relay Agent With ip Helper-Address

The ip helper-address command configures a Cisco IOS device as a DHCP relay agent on a Layer 3 interface. It then forwards broadcast UDP message to a designated host. This command cannot be used on an OOBM interface. However, it can be used on a router or switch that has Ethernet and VLAN interfaces. Layer 2 VLANs have a different IP helper-address definition, and you can use it to separate networks.

During DHCP relaying it is important that the ip-helper-address be set up on an SVI. This command enables the Layer 3 interface to receive BOOTP DHCP requests. Configure multiple ports on the switch under two SVIs. If both of them are configured as DHCP relays, the ports on the switch will automatically receive their DHCP address from both switches.

To prevent this problem, you can use DHCP snooping. This allows you to send one unicast over the network, instead of broadcasts among switches. This will reduce the overhead associated with boot-p broadcasts, and minimize the impact of malicious clients. This helps you identify the source switch from where the DHCP request originated. Usually, this is the local access switch.

Configuring a DHCP Relay Agent With ip Helper-Address
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