Content

| 1.1 |  | Overview of a Campus Network |
| 1.1.2 |  | Issues with traditional campus network designs |

The two major problems with traditional networks have always been availability and performance. These two problems are both impacted by the amount of bandwidth available. In a single collision domain, frames are visible to all devices on the LAN and are free to collide.

Multiport Layer 2 devices, such as bridges and switches, are used to segment the LAN into discrete collision domains and forward Layer 2 data frames to only the segment of the network that contains the destination address. Because the Layer 2 ports separate the LAN into distinct physical segments, they also help to resolve issues related to the distance limitations of Ethernet.

However, frames containing the broadcast Media Access Control (MAC) address are still flooded throughout the entire network. A single network device could malfunction and flood the network with "noise" and this could bring down the network. This is where routers come in. Since routers operate at Layer 3 of the Open System Interconnection (OSI) model, they are capable of making intelligent decisions regarding the flow of data to and from a network segment.

Traffic that can affect network performance is traffic that polls the network about component status or availability and advertises network component status or availability. Two common types of broadcasts that poll the network are IP Address Resolution Protocol (ARP) requests as shown in Figure and NetBIOS name requests. These broadcasts are normally propagated across an entire subnet and expect the target device to respond directly to the broadcast.

In addition to broadcasts, multicast traffic can also consume a large amount of bandwidth. Multicast traffic is propagated to a specific group of users. Depending on the number of users in a multicast group or the type of application data contained in the multicast packet, this type of broadcast can consume most, if not all, of the network resources. An example of a multicast implementation is the Cisco IP/TV solution, which uses multicast packets to transport multimedia such as audio and video.

As networks grow, so does the amount of broadcast traffic on the network. Excessive broadcasts reduce the bandwidth available to the end users and force end-user nodes to waste CPU cycles on unnecessary processes. In a worst-case scenario, broadcast storms can effectively shut down the network by monopolizing the available bandwidth.

Two methods can address the broadcast issue for large switched LAN sites. The first option is to use routers to create many subnets and logically segment the traffic, as shown in Figure . Broadcasts do not pass through routers. Although this approach will contain broadcast traffic, the CPU of a traditional router will have to process each packet. This scenario can create a bottleneck in the network.

A second option would be to implement virtual LANs (VLANs) within the switched network, as shown in Figure . For the purpose of this curriculum, VLANs are basically defined as broadcast domains. A VLAN is a group of end devices that populate multiple physical LAN segments and switch ports. They communicate as if they were on a single LAN segment. One of the primary benefits of LAN switches with VLANs is they can be used to effectively contain broadcast traffic and manage traffic flows.