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Configuring IP Addressing
Configuring Broadcast Packet Handling
IPC-34
Cisco IOS IP Configuration Guide
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-address
command 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:
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-protocol spanning-tree command 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
spanning-tree
Uses the bridging spanning-tree database to flood UDP datagrams.
Command Purpose
Router(config)# ip forward-protocol turbo-flood
Uses the bridging spanning-tree database to speed up flooding of
UDP datagrams.