Udp Helper With Subnet Broadcast Addresses - Dell Force10 S4810P Configuration Manual

1. Packet 1 is dropped at ingress if no UDP helper address is configured.
2. If UDP helper (using the command
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
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.
3. Packet 2 is also forwarded to the ingress interface with an unchanged destination address because it
does not have broadcast address configured.
Packet 1
Destination Address:
255.255.255.255
Ingress interface
IP Address: 2.1.1.1/24
UDP helper enabled
Packet 2
Switched Packet

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 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.
ip udp-helper udp-port
ip udp-broadcast-address
1/1 1/2
1/3
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
) is enabled, and the UDP destination port of
) on VLANs 100 or 101, the
VLAN 100
IP address: 1.1.0.1/24
Subnet broadcast address: 1.1.0.255
Configured broadcast address: 1.1.255.255
Hosts on VLAN 100: 1.1.0.2, 1.1.0.3, 1.1.0.4
IPv4 Routing | 533