Reference Guide

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Configurations Using UDP Helper
When you enable UDP helper and the destination IP address of an incoming packet is a broadcast address, the system
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, the system 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.
Figure 46. UDP Helper with Broadcast-All Addresses
IPv4 Routing
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