Cisco 3560G 48TS Software Configuration Guide - Page 624

Configuring IP Addressing Chapter 30 Configuring IP Unicast Routing Flooding IP Broadcasts You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface. Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment. To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.) • 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. Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to flood UDP datagrams: Step 1 Step 2 Step 3 Step 4 Step 5 Command configure terminal ip forward-protocol spanning-tree end show running-config copy running-config startup-config Purpose Enter global configuration mode. Use the bridging spanning-tree database to flood UDP datagrams. Return to privileged EXEC mode. Verify your entry. (Optional) Save your entry in the configuration file. Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts. In the Catalyst 3560 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. 30-16 Catalyst 3560 Switch Software Configuration Guide 78-16156-01