• The packet must be a MAC-level broadcast. • The packet must be an IP-level broadcast. • The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command. • The time-to-live (TTL) value of the packet must be at least two. A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.
When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface. In the switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.
Configuration Guidelines for IP Routing By default, IP routing is disabled on the device, and you must enable it before routing can take place. In the following procedures, the specified interface must be one of these Layer 3 interfaces: • A routed port: a physical port configured as a Layer 3 port by using the no switchport interface configuration command. • A switch virtual interface (SVI): a VLAN interface that is created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface. • An EtherChannel port channel in Layer 3 mode:
a port-channel logical interface that is created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group. All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. A Layer 3 switch can have an IP address that is assigned to each routed port and SVI. Note Configuring routing consists of several main procedures: • To support VLAN interfaces, create and configure VLANs on the switch or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see the "Configuring VLANs” chapter. • Configure Layer 3 interfaces. • Enable IP routing on the switch.
• Assign IP addresses to the Layer 3 interfaces. Configuring IP Unicast Routing Configuration Guidelines for IP Routing Configuring IP Unicast Routing 9