Configuration restrictions and guidelines
When you configure UDP helper, follow these restrictions and guidelines:
By default, an interface on the device does not receive directed broadcasts destined for the directly connected network. To use UDP helper, execute the ip forward-broadcast command. For more information about receiving directed broadcasts destined for the directly connected network, see "Optimizing IP performance."
Do not set UDP ports 67 and 68 for UDP helper, because UDP helper cannot forward DHCP broadcast packets.
You can specify a maximum of 256 UDP ports for UDP helper.
You can specify a maximum of 20 unicast and multicast addresses for UDP helper to convert broadcast packets on an interface.