Configuration restrictions and guidelines
When you configure UDP helper, follow these restrictions and guidelines:
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.
You can map one multicast address to a maximum of 16 broadcast and unicast addresses for UDP helper to convert multicast packets on an interface.