Reference Guide
476 | IPv4 Routing
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3. Packet 2 is also forwarded to the ingress interface with an unchanged destination address because it
does not have broadcast address configured.
Figure 24-13. UDP helper with All Broadcast 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
interfaces.
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 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 24-14. 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 .
P
acket 1
D
estination Address:
2
55.255.255.255
1/1
Ingress interface
IP Address: 2.1.1.1/24
UDP helper enabled
1/2
1/3
VLAN 100
IP address: 1.1.0.1/24
Subnet broadcast address: 1.1.0.255
Configured broadcast address: 1.1.255.2
55
Hosts on VLAN 100: 1.1.0.2, 1.1.0.3, 1.1.0.4
VLAN 101
IP address: 1.11.1/24
Subnet broadcast address: 1.1.1.255
Configured broadcast address: 1.1.255.255
Hosts on VLAN 100: 1.1.1.2, 1.1.1.3, 1.1.1.4
Packet 2
Switched Packet
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