Information About IPv4 Broadcast Packet Handling
IP Unicast Address
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 Address
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.
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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:
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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.
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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.
IP Network Broadcast
In Cisco IOS-XE devices, by default, the network prefix-directed broadcast packets are dropped in the ingress interface of a device.
In the figure, D3 has some network prefix-directed broadcast packets to be delivered through D2 to 192.168.10.x on the network. To achieve this, configure the ingress interface of D2 with the ip network- broadcast command. This enables D2 to receive and accept network prefix-directed broadcast packets.
IP Directed Broadcast Address
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
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
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:
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Cisco IOS IP Multicast Configuration Guide
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Cisco IOS IP Multicast Command Reference
Early IP Implementations
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 and IPv4 Broadcast Packets
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 Packet Forwarding
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.
UDP Broadcast Packet Flooding
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.)
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The packet must be a MAC-level broadcast (FFFF.FFFF.FFFF).
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The packet must be an IP-level broadcast (255.255.255.255).
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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.
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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.
IP Broadcast Flooding Acceleration
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.
Default UDP Port Numbers
If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:
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Time service (port 37)
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IEN-116 Name Service (port 42)
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TACACS service (port 49)
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Domain Naming System (port 53)
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BOOTP client and server packets (ports 67 and 68)
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TFTP (port 69)
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NetBIOS Name Server (port 137)
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NetBIOS Datagram Server (port 138)
Default IP Broadcast Address
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.
UDP Broadcast Packet Case Study
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.
The following sections explain the possible solutions for this application:
UDP Broadcast Packet Forwarding
The first option is UDP broadcast packet using helper addresses. To configure helper addressing, you must specify the ip helper-address command on every interface on every router that receives a UDP broadcast that needs to be forwarded. On router 1 and router 2 in the figure below, IP helper addresses can be configured to move data from the server network to the trader networks. However IP helper addressing was determined not to be an optimal solution for this type of topology because each router receives unnecessary broadcasts from the other router, as shown in the figure below.
In this case, router 1 receives each broadcast sent by router 2 three times, one for each segment, and router 2 receives each broadcast sent by router 1 three times, one for each segment. When each broadcast is received, the router must analyze it and determine that the broadcast does not need to be forwarded. As more segments are added to the network, the routers become overloaded with unnecessary traffic, which must be analyzed and discarded.
When IP helper addressing is used in this type of topology, no more than one router can be configured to forward UDP broadcasts (unless the receiving applications can handle duplicate broadcasts). This is because duplicate packets arrive on the trader network. This restriction limits redundancy in the design and can be undesirable in some implementations.
To configure routers to send UDP broadcasts bidirectionally in this type of topology, a second ip helper address command must be applied to every router interface that receives UDP broadcasts. As more segments and devices are added to the network, more ip helper address commands are required to reach them, so the administration of these routers becomes more complex over time.
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Bidirectional traffic in this topology significantly impacts router performance. |
Although IP helper addressing is well-suited to nonredundant, nonparallel topologies that do not require a mechanism for controlling broadcast loops, IP helper addressing does not work well in this topology. To improve performance, the network designers considered four other alternatives:
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Setting the broadcast address on the servers to all ones (255.255.255.255)—This alternative was dismissed because the servers have more than one interface, causing server broadcasts to be sent back onto the feed network. In addition, some workstation implementations do not allow all 1s broadcasts when multiple interfaces are present.
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Setting the broadcast address of the servers to the major network broadcast IP address--This alternative was dismissed because the TCP/IP implementation on the servers does not allow the use of major network IP broadcast addresses when the network is subnetted.
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Eliminating the subnets and letting the workstations use Address Resolution Protocol (ARP) to learn addresses—This alternative was dismissed because the servers cannot quickly learn an alternative route in the event of a primary router failure.
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UDP broadcast packet flooding—This alternative uses the spanning-tree topology created with transparent bridging to forward UDP broadcast packets in a redundant topology while avoiding loops and duplicate broadcast traffic.
UDP Broadcast Packet Flooding
UDP flooding uses the spanning-tree algorithm to forward packets in a controlled manner. Bridging is enabled on each router interface for the sole purpose of building the spanning tree. The spanning tree prevents loops by stopping a broadcast from being forwarded out an interface on which the broadcast was received. The spanning tree also prevents packet duplication by placing certain interfaces in the blocked state (so that no packets are forwarded) and other interfaces in the forwarding state (so that packets that need to be forwarded are forwarded).
Before you can enable UDP flooding, the router must be running software that supports transparent bridging and bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured for an interface, the interface will receive broadcasts, but the router will not forward those broadcasts and will not use that interface as a destination for sending broadcasts received on a different interface.
When configured for UDP flooding, the router uses the destination address specified by the ip broadcast-address command on the output interface to assign a destination address to a flooded UDP datagram. Thus, the destination address might change as the datagram propagates through the network. The source address, however, does not change.
With UDP flooding, both routers shown in the figure below use a spanning-tree to control the network topology for the purpose of forwarding broadcasts. The bridge protocol command can specify either the dec keyword (for the Digital Equipment Corporation (DEC) spanning-tree protocol) or the ieee keyword (for the IEEE Ethernet protocol). All routers in the network must enable the same spanning-tree protocol. The ip forward-protocol spanning-tree command uses the database created by the bridge protocol command. Only one broadcast packet arrives at each segment, and UDP broadcasts can traverse the network in both directions.
Because bridging is enabled only to build the spanning-tree database, use access lists to prevent the spanning-tree from forwarding non-UDP traffic.
The router configuration specifies a path cost for each interface to determine which interface forwards or blocks packets. The default path cost for Ethernet is 100. Setting the path cost for each interface on router 2 to 50 causes the spanning-tree algorithm to place the interfaces in router 2 in forwarding state. Given the higher path cost (100) for the interfaces in router 1, the interfaces in router 1 are in the blocked state and do not forward the broadcasts. With these interface states, broadcast traffic flows through router 2. If router 2 fails, the spanning-tree algorithm will place the interfaces in router 1 in the forwarding state, and router 1 will forward broadcast traffic.
With one router forwarding broadcast traffic from the server network to the trader networks, you should configure the other router to forward unicast traffic. For that reason, each router enables the ICMP Router Discovery Protocol (IRDP), and each workstation on the trader networks runs the IRDP daemon. On router 1, the preference keyword of the ip irdp command sets a higher IRDP preference than does the configuration for router 2, which causes each IRDP daemon to use router 1 as its preferred default gateway for unicast traffic forwarding. Users of those workstations can use the netstat -rn command to see how the routers are being used.
On the routers, the holdtime , maxadvertinterval , and minadvertinterval keywords of the ip irdp command reduce the advertising interval from the default so that the IRDP daemons running on the hosts expect to see advertisements more frequently. With the advertising interval reduced, the workstations will adopt router 2 more quickly if router 1 becomes unavailable. With this configuration, when a router becomes unavailable, IRDP offers a convergence time of less than one minute.
IRDP is preferred over the Routing Information Protocol (RIP) and default gateways for the following reasons:
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RIP takes longer to converge.
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Configuration of router 1 as the default gateway on each Sun workstation on the trader networks would allow those Sun workstations to send unicast traffic to router 1, but would not provide an alternative route if router 1 becomes unavailable.
The figure below shows how data flows when the network is configured for UDP flooding.
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This topology is broadcast intensive--broadcasts sometimes consume 20 percent of the 10-MB Ethernet bandwidth. However, this is a favorable percentage when compared to the configuration of IP helper addressing, which, in the same network, causes broadcasts to consume up to 50 percent of the 10-MB Ethernet bandwidth. |
If the hosts on the trader networks do not support IRDP, Hot Standby Routing Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP), or Gateway Load Balancing Protocol (GLBP) can be used to select which router will handle unicast traffic. These protocols allow the standby router to take over quickly if the primary router becomes unavailable.
Enable turbo flooding on the routers to increase the performance of UDP flooding.
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Turbo flooding increases the amount of processing that is done at interrupt level, which increases the CPU load on the router. Turbo flooding may not be appropriate on routers that are already under high CPU load or that must also perform other CPU-intensive activities. |
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