Service Manual

Configurations 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 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 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.
476
IPv4 Routing