Reference Guide

472 | IPv4 Addressing
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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 Figure 24-4, Packet 1 has the destination IP address 1.1.1.255, which matches the subnet broadcast
address of VLAN 101. If UDP helper is configured and the packet matches the specified UDP port, then
the system changes the address to the configured IP broadcast address and floods the packet on VLAN
101.
Packet 2 is sent from 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 24-4. UDP helper with Subnet Broadcast Addresses
UDP Helper with Configured Broadcast Addresses
Incoming packets with a destination IP address matching the configured broadcast address of any interface
are forwarded to the matching interfaces.
In Figure 24-5, Packet 1 has a destination IP address that matches the configured broadcast address of
VLAN 100 and 101. If UDP helper is enabled and the UDP port number matches, the packet is flooded on
both VLANs with an unchanged destination address.
Packet 2 is sent from a host on VLAN 101. It has broadcast MAC address and a destination IP address that
matches the configured broadcast address on VLAN 101. In this case, Packet 2 is flooded on VLAN 101
with the destination address unchanged because the forwarding process is Layer 2. If UDP helper is
enabled, the packet is flooded on VLAN 100 as well.
TLV 1
Chassis ID
Preamble
Start Frame
Delimiter
Destination MAC
(01:80:C2:00:00:0E)
Source MAC
Ethernet Type
(0x88CC)
LLDPDU
Padding
FCS
TLV 2
Port ID
TLV 3
Port Description
TLV 4
System Name
TLV 5
System Description
TLV 6
System Capabilities
TLV 7
Management Addr
TLV 0
End of LLDPDU
TLV 127
Organizationally Specific