An IP unicast address is not a broadcast addresses. A packet with an unicast destination IP address is intended for a specific IP host. For example, 172.16.1.1/32. Only the intended host of a unicast packets receives and processes the packet. This term is often used in conjunction with references to types of IP broadcast traffic. For example, a network administrator considering upgrading a router in a network must consider the amount of unicast, multicast, and broadcast traffic because each type of traffic can have a different effect on the performance of the router.

IP broadcast packets are sent to the destination IP broadcast address 255.255.255.255 (or the older but still occasionally used IP broadcast address of 000.000.000.000). The broadcast destination IP addresses 255.255.255.255 and 000.000.000.000 are used when a packet is intended for every IP-enabled device on a network.

Note	Packets that use the broadcast IP address as the destination IP address are known as broadcast packets.

If routers forwarded IP broadcast packets by default, the packets would have to be forwarded out every interface that is enabled for IP because the 255.255.255.255 IP destination address is assumed to be reachable via every IP enabled interface in the router. Forwarding IP broadcast packets out every interface that is enabled for IP would result in what is known as a broadcast storm (network overload due to high levels of broadcast traffic). In order to avoid the IP packet broadcast storm that would be created if a router forwarded packets with a broadcast IP destination address out every IP-enabled interface, the default behavior for a router is to not forward broadcast packets. This is a key difference between routing IP traffic at Layer 3 versus bridging it at Layer 2. Layer 2 bridges by default forward IP broadcast traffic out every interface that is in a forwarding state, which can lead to scalability problems.

Some TCP/IP protocols use the IP broadcast address to either communicate with all of the hosts on a network segment or to identify the IP address of a specific host on a network segment. For example:

• Routing Information Protocol (RIP) version 1 sends routing table information using the IP broadcast address so that any other host on the network segment running RIP version 1 can receive and process the updates.
• The Address Resolution Protocol (ARP) is used to determine the Layer 2 MAC address of the host that owns a specific Layer 3 IP address. ARP sends an IP broadcast packet (that is also a Layer 2 broadcast frame) on the local network. All of the hosts on the local network receive the ARP broadcast packet because it is sent to as a Layer 2 broadcast frame. All of the hosts on the local network process the ARP packet because it is sent to the IP broadcast address. Only the host that owns the IP address indicated in the data area of the ARP packet responds to the ARP broadcast packet.

An IP directed broadcast is intended to reach all hosts on a remote network. A router that needs to send data to a remote IP host when only the IP network address is known uses an IP directed broadcast to reach the remote host. For example, a directed broadcast sent by a host with an IP address of 192.168.100.1 with a destination IP address of 172.16.255.255 is intended only for hosts that are in the 172.16.0.0 address space (hosts that have an IP address that begins with 172.16.0.0).

An IP directed broadcast packet is routed through the network as a unicast packet until it arrives at the target subnet, where it is converted into a Layer 2 broadcast frame (MAC address of FFFF.FFFF.FFFF). Because of the nature of the IP addressing architecture, only the last router in the chain, the one that is connected directly to the target subnet, can conclusively identify a directed broadcast. For example, only a router with an interface connected to a network using an IP address in the 172.16.0.0/16 address space such as 172.16.1.1/16 can determine that a packet sent to 172.16.255.255 is a directed broadcast and convert it to a Layer 2 broadcast that is received by all hosts on the local network. The other routers in the network that are not connected to the 172.16.0.0/16 network forward packets addressed to 172.16.255.255 as if they were for a specific IP host.

All of the hosts on the remote network receive IP directed broadcasts after they are converted to Layer 2 broadcast frames. Ideally only the intended destination host will fully process the IP directed broadcast and respond to it. However, IP directed broadcasts can be used for malicious purposes. For example, IP directed broadcasts are used in "smurf" Denial of Service (DoS) attack and derivatives thereof. In a "smurf" attack, the attacker sends Internet Control Message Protocol (ICMP) echo requests (pings) to a directed broadcast address using the source IP address of the device that is the target of the attack. The target is usually a host inside a company’s network such as a web server. The ICMP echo requests are sent to an IP directed broadcast address in the company’s network that causes all the hosts on the target subnet to send ICMP echo replies to the device under attack. By sending a continuous stream of such requests, the attacker can create a much larger stream of replies, which can completely inundate the host that is under attack. For information on how IP directed broadcasts are used in DoS attacks, search the Internet for "IP directed broadcasts," "denial of service," and "smurf attacks."

Due to the security implications of allowing a router to forward directed broadcasts and the reduction in applications that require directed broadcasts, IP directed broadcasts are disabled by default in Cisco IOS Release 12.0 and later releases. If your network requires support for IP directed broadcasts, you can enable it on the interfaces that you want to translate the IP directed broadcasts to Layer 2 broadcasts using the ip directed-broadcast command. For example, if your router is receiving IP directed broadcasts on Fast Ethernet interface 0/0 for the network address assigned to Fast Ethernet interface 0/1, and you want the IP directed broadcasts to be translated to Layer 2 broadcasts out interface Fast Ethernet interface 0/1, configure the ip directed-broadcast command on Fast Ethernet interface 0/1. You can specify an access list to control which IP directed broadcasts are translated to Layer 2 broadcasts. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to Layer 2 broadcasts. For example, if you know that the only legitimate source IP address of any IP directed broadcasts in your network is 192.168.10.2, create an extended IP access list allowing traffic from 192.168.10.2 and assign the access list with the ip directed-broadcastaccess-list command.

IP directed broadcasts are dropped by default. Dropping IP directed broadcasts reduces the risk of DoS attacks.

You can enable forwarding of IP directed broadcasts on an interface where the broadcast becomes a physical broadcast. You enable the translation of directed IP broadcast packets to Layer 2 broadcast frames on the interface that is connected to the IP network that the IP directed broadcast is addressed to. For example, if you need to translate IP directed broadcasts with the IP destination address of 172.16.10.255 to Layer 2 broadcast frames, you enable the translation on the interface that is connected to IP network 172.16.10.0/24.

You can specify an access list to control which directed broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts.

IP directed broadcasts are disabled by default in Cisco IOS Release 12.0 and newer releases.

IP multicast addresses are intended to reach an arbitrary subset of the hosts on a local network. IP broadcast addresses create a problem because every host must receive and process the data in each packet to determine if it contains information that the host must process further. IP multicast addresses resolve this problem by using well-known IP addresses that a host must be configured to recognize before it will process packets addressed to it. When a host receives an IP multicast packet, the host compares the IP multicast address with the list of multicast addresses it is configured to recognize. If the host is not configured to recognize the IP multicast address, the host ignores the packet instead of processing it further to analyze the data in the packet. Because the host can ignore the packet it spends less time and fewer resources than it would have had to spend if the packet had been an IP broadcast that had to be processed all the way to the data layer before it was discarded.

The range of IP addresses reserved for Class D multicast addresses is 224.0.0.0 to 239.255.255.255/32 (255.255.255.255).

Most of the TCP/IP routing protocols use IP multicast addresses to send routing updates and other information to hosts on the same local network that are running the same routing protocol. Many other applications such as audio/video streaming over the Internet use IP multicast addresses. For a list of the currently assigned IP multicast addresses see Internet Multicast Addresses.

Information on configuring network devices for IP multicast support is available in the following documentation:

• Cisco IOS IP Multicast Configuration Guide
• Cisco IOS IP Multicast Command Reference

Several early IP implementations do not use the current broadcast address standard of 255.255.255.255. Instead, they use the old standard, which calls for all zeros (000.000.000.000) instead of all ones to indicate broadcast addresses. Many of these implementations do not recognize an all-1s broadcast address and fail to respond to the broadcast correctly. Others forward all-1s broadcasts by default, which causes a serious network overload known as a broadcast storm. Implementations that exhibit these problems include systems based on versions of Berkeley Standard Distribution (BSD) UNIX prior to Version 4.3.

DHCP requires that the client (host requiring information from the DHCP server) send broadcast packets to find a DHCP server to request configuration information from. If the DHCP server is not on the same network segment as the client that is sending the DHCP broadcasts, the router must be configured to forward the DHCP requests to the appropriate network.

For more information on DHCP, see RFC 2131 Dynamic Host Configuration Protocol, at http://www.ietf.org/rfc/rfc2131.txt.

UDP broadcast packets are used by TCP/IP protocols such as DHCP and applications that need to send the same data to multiple hosts concurrently. Because routers by default do not forward broadcast packets you need to customize your router’s configuration if your network has UDP broadcast traffic on it. One option for forwarding UDP broadcast packets is to use the UDP forwarding feature. UDP forwarding rewrites the broadcast IP address of a UDP packet to either a unicast (specific host) IP address or a directed IP broadcast. After the address is rewritten the UDP packet is forwarded by all of the routers in the path to the destination network without requiring additional configuration changes on the other routers.

You can enable forwarding of UDP broadcast packets, such as DHCP requests, to a host, or to multiple hosts on the same target network. When a UDP broadcast packet is forwarded, the destination IP address is rewritten to match the address that you configure. For example, the ip helper-address 172.16.10.2 command rewrites the IP destination address from 255.255.255.255 to 172.16.10.2.

To enable UDP broadcast packet forwarding to specific host, use a specific host IP address as the helper address when you configure the ip helper-address address command. To enable UDP broadcast packet forwarding to a range of hosts to allow for load sharing and redundancy, use an IP directed broadcast address as the helper address when you configure the ip helper-address address command.

You can allow IP broadcasts to be flooded throughout your network in a controlled fashion using the database created by theLayer 2 bridging Spanning Tree Protocol (STP). Enabling this feature also prevents flooding loops. In order to support this capability, the Cisco IOS software on your router must include support for transparent bridging, and transparent bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, the interface is still able to receive broadcasts. However, the interface will never forward broadcasts it receives, and the router will never use 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.

In order to be considered for flooding, packets must meet the following criteria. (These are the same conditions used to consider packet forwarding using IP helper addresses.)

• The packet must be a MAC-level broadcast (FFFF.FFFF.FFFF).
• The packet must be an IP-level broadcast (255.255.255.255).
• The packet must be a Trivial File Transfer Protocol (TFTP), Domain Name System (DNS), Time, NetBIOS, Neighbor Discovery (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.

If you want to send the flooded UDP packets to a specific host, you can change the Layer 3 IP broadcast address of the flooded UDP packets with the ip broadcast-address command in interface configuration mode. The address of the flooded UDP packets can be set to any desired IP address. The source address of the flooded UDP packet is never changed. The TTL value of the flooded UDP packet is decremented.

After a decision has been made to send the datagram out on an interface (and the destination IP 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.

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" module 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.

You can accelerate 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 high-level data link control (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.

If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:

• Time service (port 37)
• IEN-116 Name Service (port 42)
• TACACS service (port 49)
• Domain Naming System (port 53)
• BOOTP client and server packets (ports 67 and 68)
• TFTP (port 69)
• NetBIOS Name Server (port 137)
• NetBIOS Datagram Server (port 138)

The Cisco IOS software supports sending IP broadcasts on both LANs and WANs. There are several ways to indicate an IP broadcast address. The default is an address consisting of all ones (255.255.255.255), although the software can be configured to generate any form of IP broadcast address such as all zeros (0.0.0.0), and directed broadcasts such as 172.16.255.255. Cisco IOS software can receive and process most IP broadcast addresses.

This case study is from a trading floor application in a financial company. The workstations (WS1, WS2, and WS3) in the following figure receive financial data from the feed network. The financial data is sent using UDP broadcasts.

Figure 1

Topology that Requires UDP Broadcast Forwarding

The following sections explain the possible solutions for this application:

• UDP Broadcast Packet Forwarding
  
• UDP Broadcast Packet Flooding
  

1.    enable

2.    configure terminal

3.    interface type number

4.    ip address address mask

5.    ip directed-broadcast

6.    end

1.    enable

2.    configure terminal

3.    access-list 100-199 permit ip source-address mask destination-address mask

4.    interface type number

5.    ip address address mask

6.    ip directed-broadcast access-list

7.    end

1.    enable

2.    configure terminal

3.    ip forward-protocol udp

4.    interface type number

5.    ip address address mask

6.    ip helper-address address

7.    end

1.    enable

2.    configure terminal

3.    ip forward-protocol udp

4.    interface type number

5.    ip address address mask

6.    ip helper-address address

7.   exit

8.    interface type number

9.    ip address address mask

10.    ip directed-broadcast

11.    end

If you router does not have NVRAM, and you need to change the IP broadcast address to 0.0.0.0, you must change the IP broadcast address manually by setting jumpers in the processor configuration register. Setting bit 10 causes the device to use all 0s. Bit 10 interacts with bit 14, which controls the network and host portions of the broadcast address. Setting bit 14 causes the device to include the network and host portions of its address in the broadcast address. The table below shows the combined effect of setting bits 10 and 14.

For additional information on setting the hardware jumpers on your router, see the hardware documentation that was supplied with you router.

Cisco IOS-based routers with NVRAM have software configuration registers that allow you to modify several behaviors of the router such as where it looks for images to load, what IP broadcast address it uses, and the console line speed. The factory default value for the configuration register is 0x2102 where 0X indicates this a hexadecimal number. The config-register command is used to modify the settings of the software configuration registers.

Information on configuring other behaviors with the software configuration registers using the config-register command is available in the following documentation:

• "Loading and Managing System Images" chapter of the Cisco IOS Configuration Fundamentals Configuration Guide
• Cisco IOS Configuration Fundamentals Command Reference

Caution

You need to be very careful when you change the software configuration registers on your router because if you inadvertently alter the console port line speed, you will not be able to configure the router with a terminal server on the console port unless you know the speed that you set for the console port, and you know how to change the line speed for your terminal application. If your router is configured for alternate access to the CLI such as using Telnet or a web browser, you can use this method to log in to the router and change the software configuration register back to 0x2102.

Perform this task to set the IP broadcast address on every interface to 0.0.0.0 while maintaining the remainder of the default values for the software configuration register settings.

1.    enable

2.    configure terminal

3.    config-register value

4.    end

1.    enable

2.    configure terminal

3.    interface type number

4.    ip address address mask

5.    ip broadcast-address address

6.    end

1.    enable

2.    configure terminal

3.    bridge number protocol ieee

4.    ip forward-protocol spanning-tree

5.    ip forward-protocol turbo-flood

6.    ip forward-protocol udp

7.    interface type number

8.    ip address address mask

9.    bridge-group number

10.    interface type number

11.    ip address address mask

12.    bridge-group number

13.    end