Concept Guide

Important Points to Remember
• The existing ip directed broadcast command is rendered meaningless if you enable UDP helper on the same interface.
• The broadcast trac rate should not exceed 200 packets per second when you enable UDP helper.
• You may specify a maximum of 16 UDP ports.
• UDP helper is compatible with IP helper (ip helper-address):
– UDP broadcast trac with port number 67 or 68 are unicast to the dynamic host conguration protocol (DHCP) server per the ip
helper-address conguration whether or not the UDP port list contains those ports.
– If the UDP port list contains ports 67 or 68, UDP broadcast trac is forwarded on those ports.
Enabling UDP Helper
To enable UDP helper, use the following command.
• Enable UPD helper.
ip udp-helper udp-ports
Example of Enabling UDP Helper and Using the UDP Helper show Command
To view the interfaces and ports on which you enabled UDP helper, use the show ip udp-helper command from EXEC Privilege
mode.
Congurations Using UDP Helper
When you enable UDP helper and the destination IP address of an incoming packet is a broadcast address, Dell EMC Networking OS
suppresses the destination address of the packet.
The following sections describe various congurations that employ UDP helper to direct broadcasts.
• UDP Helper with Broadcast-All Addresses
• UDP Helper with Subnet Broadcast Addresses
• UDP Helper with Congured Broadcast Addresses
• UDP Helper with No Congured Broadcast Addresses
UDP Helper with Broadcast-All Addresses
When the destination IP address of an incoming packet is the IP broadcast address, Dell EMC Networking OS rewrites the address to
match the congured broadcast address.
In the following illustration:
1 Packet 1 is dropped at ingress if you did not congure UDP helper address.
2 If you enable UDP helper (using the ip udp-helper udp-port command), and the UDP destination port of the packet matches
the UDP port congured, the system changes the destination address to the congured broadcast 1.1.255.255 and routes the packet
to VLANs 100 and 101. If you do not congure an IP broadcast address (using the
ip udp-broadcast-address command) on
VLANs 100 or 101, the packet is forwarded using the original destination IP address 255.255.255.255.
Packet 2, sent from a host on VLAN 101 has a broadcast MAC address and IP address. In this case:
1 It is ooded on VLAN 101 without changing the destination address because the forwarding process is Layer 2.
2 If you enabled UDP helper, the system changes the destination IP address to the congured broadcast address 1.1.255.255 and
forwards the packet to VLAN 100.
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IPv4 Routing