Concept 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 congured, the system changes the destination address to the congured broadcast 1.1.255.255 and routes the packet
to VLANs 100 and 101. If you do not congure 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 ooded 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 congured 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 congured.
Figure 44. 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 congured 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 congured UDP helper and the packet matches the specied UDP port, the system changes the address to the congured IP
broadcast address and oods 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
ooded on VLAN 101 in its original condition as the forwarding process is Layer 2.
Figure 45. UDP Helper with Subnet Broadcast Addresses
IPv4 Routing
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