IPv4 Addressing | 479UDP Helper with Broadcast-all AddressesWhen the destination IP address of an incoming packet is the IP broadcast address, FTOS rewrites theaddress to match the configured broadcast address.In Figure 21-14:1. Packet 1 is dropped at ingress if no UDP helper address is configured.2. If UDP helper (using the command ip udp-helper udp-port) is enabled, and the UDP destination port ofthe packet matches the UDP port configured, the system changes the destination address to theconfigured broadcast 1.1.255.255 and routes the packet to VLANs 100 and 101. If an IP broadcastaddress is not configured (using the command ip udp-broadcast-address) on VLANs 100 or 101, thepacket is forwarded using the original destination 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 isLayer 2.2. If UDP helper is enabled, the system changes the destination IP address to the configured broadcastaddress 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 itdoes not have broadcast address configured.Figure 21-14. UDP helper with All Broadcast AddressesUDP Helper with Subnet Broadcast AddressesWhen the destination IP address of an incoming packet matches the subnet broadcast address of anyinterface, the system changes the address to the configured broadcast address and sends it to matchinginterface.Packet 1Destination Address:255.255.255.2551/1Ingress interfaceIP Address: 2.1.1.1/24UDP helper enabled1/21/3VLAN 100IP address: 1.1.0.1/24Subnet broadcast address: 1.1.0.255Configured broadcast address: 1.1.255.255Hosts on VLAN 100: 1.1.0.2, 1.1.0.3, 1.1.0.4VLAN 101IP address: 1.11.1/24Subnet broadcast address: 1.1.1.255Configured broadcast address: 1.1.255.255Hosts on VLAN 100: 1.1.1.2, 1.1.1.3, 1.1.1.4Packet 2Switched Packet
