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 49. UDP Helper with Broadcast-All Addresses
UDP Helper with Subnet Broadcast Addresses
When the destination IP address of an incoming packet matches the subnet broadcast address of any
interface, the system changes the address to the configured broadcast address and sends it to matching
interface.
In the following illustration, Packet 1 has the destination IP address 1.1.1.255, which matches the subnet
broadcast address of VLAN 101. If you configured UDP helper and the packet matches the specified UDP
port, the system changes the address to the configured IP broadcast address and floods the packet on
VLAN 101.
Packet 2 is sent from the host on VLAN 101. It has a broadcast MAC address and a destination IP address
of 1.1.1.255. In this case, it is flooded on VLAN 101 in its original condition as the forwarding process is
Layer 2.
Figure 50. UDP Helper with Subnet Broadcast Addresses
496
IPv4 Routing
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