4. VLANs And Trunking - Packet Guide To Routing And Switching ...

Problem: Big Broadcast Domains

With any single shared media LAN segment, transmissions propagate through the entire segment. As traffic activity increases, more collisions occur and transmitting nodes must back off and wait before attempting the transmission again. While the collision is cleared, other nodes must also wait, further increasing congestion on the LAN segment.

The left side of Figure 4-1 depicts a small network in which PC 2 and PC 4 attempt transmissions at the same time. The frames propagate away from the computers, eventually colliding with each other somewhere in between the two nodes as shown on the right. The increased voltage and power then propagate away from the scene of the collision. Note that the collision does not continue past the switches on either end. These are the boundaries of the collision domain. This is one of the primary reasons for switches replacing hubs. Hubs (and access points) simply do not scale well as network traffic increases.

Figure 4-1. Before and after collision

The use of switches at Layer 2 eliminates much of the scaling problem because they filter out problems such as collisions. Instead, transmissions are now governed by the behavior of the switches and the broadcast domain. A broadcast domain defines the area over which a broadcast frame will propagate. For example, an ARP request issued by PC 3 results in a broadcast frame that propagates through the switches all the way to the routers as shown in Figure 4-2. A broadcast frame has the broadcast address (FF-FF-FF-FF-FF-FF) as the destination MAC.

Figure 4-2. Broadcast domain

With the improved performance and filtering resulting from the use of switches, there is a temptation to create large Layer 2 topologies and add lots of nodes, but this creates a large broadcast domain. The problem is that all devices on a network (computers, printers, switching equipment, etc.) generate broadcast and multicast frames that traverse the entire broadcast domain, competing with data traffic for bandwidth. Much of this traffic is for management of the network and includes protocols for address resolution (ARP), dynamic host configuration (DHCP), spanning tree (STP), and an assortment of Windows tasks. Figure 4-3 illustrates the potential difficulty. Assume that PC1 has generated the following requests: ARP, Windows registration, and DHCP.

Figure 4-3. Broadcast frame growth

Because all of the requests use a broadcast frame, as they are received at Switch 1, the frames are forwarded in all directions. As the other switches in the topology follow suit, the frames traverse the entire network and are received at all other nodes and the routers.

As the number of network nodes increases, the amount of overhead also increases. Each switch might be connected to dozens of nodes, with each node generating the several broadcast frames. If enough traffic is created, even a switched network can have poor performance. Deploying VLANs can help solve this problem by breaking up the broadcast domain and separating the traffic.