UDP Broadcast Flooding

www.net130.com 日期：2005-6-6 浏览次数：
出处：Cisco网站

Implementing IP Helper Addressing

IP helper addressing is a form of static addressing that uses directed broadcasts to forward local and all-nets broadcasts to desired destinations within the internetwork.

To configure helper addressing, you must specify the ip helper-address command on every interface on every router that receives a broadcast that needs to be forwarded. On Router A and Router B, IP helper addresses can be configured to move data from the TIC server network to the trader networks. IP helper addressing in not the optimal solution for this type of topology because each router receives unnecessary broadcasts from the other router, as shown in Figure 19-2.

Figure 19-2: Flow of UDP packets from routers to trader networks using IP helper addressing.

In this case, Router A receives each broadcast sent by Router B three times, one for each segment, and Router B receives each broadcast sent by Router A 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 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. Note, too, that 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, in view of these drawbacks, IP helper addressing does not work well in this topology. To improve performance, network designers considered several other alternatives:

Setting the broadcast address on the TIC servers to all ones (255.255.255.255)—This alternative was dismissed because the TIC servers have more than one interface, causing TIC broadcasts to be sent back onto the Feed network. In addition, some workstation implementations do not allow all ones broadcasts when multiple interfaces are present.
Setting the broadcast address of the TIC servers to the major net broadcast (164.53.0.0)—This alternative was dismissed because the Sun TCP/IP implementation does not allow the use of major net broadcast addresses when the network is subnetted.
Eliminating the subnets and letting the workstations use Address Resolution Protocol (ARP) to learn addresses—This alternative was dismissed because the TIC servers cannot quickly learn an alternative route in the event of a primary router failure.

With alternatives eliminated, the network designers turned to a simpler implementation that supports redundancy without duplicating packets and that ensures fast convergence and minimal loss of data when a router fails: UDP flooding.

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