Service Manual

Congurations Using UDP Helper
When you enable UDP helper and the destination IP address of an incoming packet is a broadcast address, Dell 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 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.
3. Packet 2 is also forwarded to the ingress interface with an unchanged destination address because it does not have broadcast
address congured.
Figure 43. UDP Helper with Broadcast-All Addresses
IPv4 Routing
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