Cisco Systems 78-11741-02 manual Speeding Up Flooding of UDP Datagrams, IPC-34

Configuring IP Addressing

Configuring Broadcast Packet Handling

In order to be considered for flooding, packets must meet the following 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 Trivial File Transfer Protocol (TFTP), DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP protocol 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 you specified with the ip broadcast-addresscommand in the interface configuration mode on the output interface. The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

After a decision has been made to send the datagram out on 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.

To use the bridging spanning-tree database to flood UDP datagrams, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip forward-protocol	Uses the bridging spanning-tree database to flood UDP datagrams.
spanning-tree

If no actual bridging is desired, you can configure a type-code bridging filter that will deny all packet types from being bridged. Refer to the “Configuring Transparent Bridging” chapter of the Cisco IOS Bridging and IBM Networking Configuration Guide for more information about using access lists to filter bridged traffic. The spanning-tree database is still available to the IP forwarding code to use for the flooding.

Speeding Up Flooding of UDP Datagrams

You can speed up flooding of UDP datagrams using the spanning-tree algorithm. Used in conjunction with the ip forward-protocolspanning-treecommand in global configuration mode, this feature boosts the performance of spanning tree-based UDP flooding by a factor of about four to five times. The feature, called turbo flooding, is supported over Ethernet interfaces configured for Advanced Research Projects Agency (ARPA) encapsulated, FDDI, and HDLC-encapsulated serial interfaces. However, it is not supported on Token Ring interfaces. As long as the Token Rings and the non-HDLC serial interfaces are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.

To enable turbo flooding, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip forward-protocol turbo-flood	Uses the bridging spanning-tree database to speed up flooding of
	UDP datagrams.

Cisco IOS IP Configuration Guide

IPC-34