Reference Guide

IPv4 Routing | 475
Figure 24-12. Configuring a Broadcast Address
Configurations Using UDP Helper
When UDP helper is enabled 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 illustration below:
1. Packet 1 is dropped at ingress if no UDP helper address is configured.
2. If UDP helper (using the command
ip udp-helper udp-port) is enabled, 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 an IP broadcast
address is not configured (using the command
ip udp-broadcast-address) 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 UDP helper is enabled, the system changes the destination IP address to the configured broadcast
address 1.1.255.255 and forwards the packet to VLAN 100.
Dell(conf)#do show interfaces vlan 100
Vlan 100 is up, line protocol is down
Address is 00:01:e8:0d:b9:7a, Current address is 00:01:e8:0d:b9:7a
Interface index is 1107787876
Internet address is 1.1.0.1/24
IP UDP-Broadcast address is 1.1.255.255
MTU 1554 bytes, IP MTU 1500 bytes
LineSpeed auto
ARP type: ARPA, ARP Timeout 04:00:00
Last clearing of "show interface" counters 00:07:44
Queueing strategy: fifo
Input Statistics:
0 packets, 0 bytes
Time since last interface status change: 00:07:44