A broadcast is a data packet destined for all hosts on a network.
|
Broadcasts are identified by a broadcast address.
|
A broadcast address is a special address reserved for sending a message to all stations.
|
The IP network administrator is responsible for controlling broadcast messages on the network.
|
Cisco IOS 11.1 supports IP broadcasts on both LANs and WANs.
|
It can be configured to generate any form of IP broadcast address.
|
The default is an address consisting of four octets of ones (255.255.255.255).
|
Cisco IOS 11.1 supports two types of broadcasting:

• directed broadcasting
|
• flooding
|
A directed broadcast is a packet sent to a specific network or series of networks.
|
A flooded broadcast packet is sent to every network.
|
A directed broadcast address includes the network or subnet fields.
|
For example, if the network address is 172.16.0.0, the address 172.16.255.255 indicates all hosts on network 172.16.0.0.
|
This would be a directed broadcast.
|
If network 172.16.0.0 has a subnet mask of 255.255.255.0 (the third octet is the subnet field), the address 172.16.5.255 specifies all hosts on subnet 5 of network 172.16.0.0, another directed broadcast.
|
Broadcast packets can cause a serious network overload known as a broadcast storm.
|
A broadcast storm is an  undesirable network event in which many broadcasts are sent simultaneously across all network segments.
|
The old broadcast standard uses four octets of all zeros instead of all ones to indicate broadcast addresses.
|
Many implementations using this standard do not recognize an all-ones broadcast address and fail to respond to the broadcast correctly.
|
Others forward all-ones broadcasts, resulting in a serious network overload.
|
Such broadcast storms use substantial network bandwidth and, typically, cause network time-outs.
|
Routers do not forward broadcast packets, by default.
|
They provide some protection from broadcast storms by limiting their extent to the local cable.
|
However, bridges will propagate broadcast storms, as they are OSI Layer 2 devices which forward broadcasts to all network segments.
@@
The best solution to broadcast storms is to use a single broadcast address scheme on a network.
|
Modern IP implementations allow the network administrator to set the address used as the broadcast address.
|
Cisco IOS 11.1 accepts and interprets all possible forms of broadcast address.
|
The current broadcast standard provides specific addressing schemes for forwarding broadcasts.
|
The IP Broadcast-Address command allows you to specify the IP broadcast address for your network.
|
This takes the specified IP address as its only argument.
|
When you wish to specify the IP broadcast address on a router without nonvolatile memory (NVRAM), you need to set jumpers in the processor configuration register (Bit 10 and Bit 14).
|
Consider an interface where directed broadcasts are translated to physical broadcasts.
|
You can use the IP Directed-Broadcast command to enable "directed broadcast-to-physical broadcast" translation on the interface.
|
Forwarding of directed broadcasts on this type of interface is enabled, by default, for those protocols configured using the IP Forward-Protocol global configuration command.
|
You have the option of defining an access list to control which broadcasts are forwarded.
|
Only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts.
|
You provide the access list number as the optional argument to the IP Directed-Broadcast command.
@@
You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion.
|
This feature also helps prevent routing loops.
|
Broadcast traffic received on an interface is sent out from all interfaces on that device, except the interface on which the information was originally received.
|
In the graphic, the broadcast packet arriving on interface E0 is flooded to interfaces E1, E2, and S0.
|
Broadcast flooding can be done in a controlled manner using the database created by the Bridging Spanning-Tree Protocol.
|
A spanning-tree is a loop-free subset of a network topology.

The Spanning-Tree Protocol is a Bridge protocol that utilizes the spanning-tree algorithm, enabling a learning bridge to dynamically work around loops in a network topology by creating a spanning tree.

Bridges exchange hello messages (Bridge Protocol Data Units - BPDUs) with other bridges to detect loops, and then remove the loops by shutting down selected bridge interfaces.
|
Cisco IOS 11.1 provides the IP Forward-Protocol Spanning-Tree command to achieve flooding of UDP datagrams using the bridging spanning-tree database.
|
Bridging must be configured on each interface that is to participate in the flooding.
|
In addition, the routing software must include transparent bridging.
|
Transparent bridging refers to a scheme where bridges pass frames along one hop at a time, based on tables which associate end nodes with bridge ports.

Transparent bridging is so named because the presence of bridges is transparent to network end nodes.
|
If bridging is not configured on an interface, it can receive broadcasts but can never forward them.
|
Packets must meet all the following criteria before they can be considered candidates for flooding:

• the packet must be a 
  MAC-level broadcast
|
• the packet must be an 
  IP-level broadcast (not 
  all MAC-level broadcasts 
  are IP broadcasts)
|
• the packet must be a 
  TFTP, DNS, NetBIOS, ND, or 
  BOOTP packet, or a UDP 
  protocol specified by the IP 
  Forward-Protocol command
|
These are the most common users of the broadcast feature and the ones you may want to flood.
|
• the packet's time-to-live 
  (TTL) value must be two or 
  greater
|
A flooded UDP packet is given the destination address that was specified with the IP Broadcast-Address command on the output interface.
|
The destination address may change as the packet propagates through the network, but the source address remains constant.
|
The TTL value is also decremented as propagation continues.
|
The broadcast packet is handed to the normal IP output routines once the decision has been made to send it out on an interface.
|
The broadcast packet is therefore subject to whatever filters and access lists that are defined on the output interface.
|
Flooding of UDP packets based on the bridging spanning-tree database is implemented using the IP Forward-Protocol Spanning-Tree command.
|
The spanning-tree algorithm can be used to significantly speed-up spanning-tree-based UDP flooding.
|
This feature is called £Rturboflooding.£r
|
You can enable it using the IP Forward-Protocol Turbo-Flood global configuration command.
|
Turboflooding is supported over Ethernet interfaces configured for ARPA encapsulated, Fiber Distributed Data Interface (FDDI), and HDLC-encapsulated serial interfaces.
|
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 used for UDP flooding, turboflooding will behave normally.
@@
Network hosts can use User Datagram Protocol (UDP) broadcasts to determine address, configuration, and name information.
|
If the network host is on a segment that has no server, UDP broadcasts are normally not forwarded.
|
You can overcome this problem by configuring a £Rhelper address£r on the interface where broadcasts are expected.
|
The helper address is the address to which selected classes of broadcasts received on an interface will be sent.
|
Helper address configuration provides proper connectivity by forwarding broadcast traffic that might not otherwise transit the network.
|
You can configure a number of helper addresses on each interface.
|
Where a helper address has been defined for an interface, UDP and ND (Network Disk) forwarding are enabled by default.
|
Cisco IOS gives you the option of specifying a UDP destination port to control which UDP services are forwarded.
|
You are allowed to specify multiple UDP protocols.
|
If you configure the forwarding of UDP broadcasts without specifying a UDP port, you  effectively configure the router to act as a BOOTP forwarding agent.
|
BOOTP is a protocol used by diskless workstations to discover their IP address, the address of a server host, and the name of a file to be loaded into memory and executed at boot time.
|
BOOTP packets can carry Dynamic Host Configuration Protocol (DHCP) information.
|
This makes Cisco IOS compatible with DHCP clients.
|
Broadcast 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 over each network segment.
@@
Let's consider the IP helper address example presented in the graphic.
|
The IP Forward-Protocol command specifies which class of broadcast packet will be forwarded.
|
Here, the decision is to forward UDP broadcasts.
|
No UDP port is specified, so the router will act as a BOOTP forwarding agent.
|
For this example, broadcasts are expected on the Ethernet 0 interface.
|
Therefore, this is the interface on which to configure helper addresses.
|
The IP Helper-Address command accomplishes the following:

• it enables forwarding
|
• it specifies the destination 
  address for UDP broadcast 
  packets
|
This effectively changes a destination address from a broadcast to a unicast address.
|
Here, two helper addresses are configured on the Ethernet 0 interface of the main router.
|
Incoming UDP broadcast packets received on this interface are to be forwarded to host addresses 172.16.1.2 and 172.16.3.4.
|
Subnetting is in operation in this example, with the third portion of the address acting as the subnet identifier.
|
Thus, helper addressing provides a simple mechanism for controlling how broadcast traffic flows within and between networks.
@@
