计算机局域网技术详解.ppt

* * * * * * * * * * * * * Purpose: Explore one of the ways to limit broadcast traffic within the entire network. Emphasize: Because broadcasts are recognized by all hosts, a significant goal of router configuration is to control the unnecessary proliferation of broadcast packets. Routers are used to frame Layer 2 broadcast domains. A router calculates routes and forwards traffic only to the node containing the correct network address, such as its IP or IPX address. This allows each workgroup to have much more bandwidth, prevents broadcast traffic from flooding the entire network, and provides some security. For IP traffic, users are divided into different IP address subnets that are connected at Layer 3 by routers. Traffic using Layer 2 Media Access Control (MAC) addressing is constrained to stay within the subnet. However, routers do not prevent all broadcast traffic. Typical examples of broadcast traffic that is propagated between routers are the routing updates and Novell SAP updates that are broadcast between routers. An excessively high population of routers in any area of the overall network might result in bottlenecks brought on by this type of broadcast replication. Transition: The following describes the use of VLANS to define broadcast domains * Purpose: Cover another method for controlling broadcast traffic. Note: The students should be familiar with basic VLAN concepts. These concepts were covered in the “Interconnecting Cisco Network Devices,” course which is a prerequisite to this course. However, you may want to discuss the intent of VLANS. Emphasize: A VLAN can be equated to a broadcast domain. VLANs can be seen as analogous to a group of end stations, perhaps on multiple physical LAN segments, that are not constrained by their physical location and can communicate as if they were on a common LAN. VLANs represent an alternative solution to routers for broadcast containment, since VLANs allow switches to also contain broadcast traffic. With the im