IPv4 Routing | 273Figure 14-11. Viewing the UDP Broadcast ConfigurationConfigurations Using UDP HelperWhen you enable UDP helper and the destination IP address of an incoming packet is a broadcast address,FTOS suppresses the destination address of the packet. The following sections describe variousconfigurations that employ UDP helper to direct broadcasts.• UDP Helper with Broadcast-All Addresses• UDP Helper with Subnet Broadcast Addresses• UDP Helper with Configured Broadcast Addresses• UDP Helper with No Configured Broadcast AddressesUDP 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 14-12:1. Packet 1 is dropped at ingress if no UDP helper address is configured.2. If you enable UDP helper (using the ip udp-helper udp-port command), 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 ip udp-broadcast-address command) on VLANs 100 or 101, thepacket 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 isLayer 2.2. If you enable UDP helper, 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.FTOS#show ip udp-helper--------------------------------------------------Port UDP port list--------------------------------------------------TenGig 1/1 1000
