Users Guide

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 52. 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 53. UDP Helper with Subnet Broadcast Addresses
IPv4 Routing
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