Administrator Guide

○ UDP broadcast traffic with port number 67 or 68 are unicast to the dynamic host configuration protocol (DHCP) server
per the ip helper-address configuration whether or not the UDP port list contains those ports.
○ If the UDP port list contains ports 67 or 68, UDP broadcast traffic is forwarded on those ports.
Enabling UDP Helper
To enable UDP helper, use the following command.
● Enable UPD helper.
ip udp-helper udp-ports
DellEMC(conf-if-gi-1/1)#ip udp-helper udp-port 1000
DellEMC(conf-if-gi-1/1)#show config
!
interface GigabitEthernet 1/1
ip address 2.1.1.1/24
ip udp-helper udp-port 1000
no shutdown
To view the interfaces and ports on which you enabled UDP helper, use the show ip udp-helper command from EXEC
Privilege mode.
DellEMC#show ip udp-helper
--------------------------------------------------
Port UDP port list
--------------------------------------------------
Gi 1/1 1000
Configurations 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 configurations that employ UDP helper to direct broadcasts.
● UDP Helper with Broadcast-All Addresses
● UDP Helper with Subnet Broadcast Addresses
● UDP Helper with Configured Broadcast Addresses
● UDP Helper with No Configured 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 configured broadcast address.
In the following illustration:
1. Packet 1 is dropped at ingress if you did not configure 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 configured, the system changes the destination address to the configured broadcast 1.1.255.255 and
routes the packet to VLANs 100 and 101. If you do not configure 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 flooded 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 configured broadcast address 1.1.255.255
and forwards the packet to VLAN 100.
3. Packet 2 is also forwarded to the ingress interface with an unchanged destination address because it does not have
broadcast address configured.
370
IPv4 Routing