2.4 Determining Bandwidth Needs
2.4.5 Determining the distribution-layer requirements
In this example, the distribution layer must be capable of providing the following capacity: 
  • Total load at the distribution-layer switch is the number of access switches x 96 Mbps. In this scenario, there are ten access switches, or 10 x 96 Mbps, yielding a 960-Mbps aggregate bandwidth requirement at the distribution layer. 
  • Eighty percent of the traffic is local to the switch block and is not routed across the core. 
  • Twenty percent of the traffic is remote and is routed toward the core. 
  • Taking into consideration that only 20 percent of traffic is remote, we come up with 20 percent x 960 Mbps, yielding 192 Mbps of traffic that must be able to cross the core. 

This sample network supports a redundant core; therefore, each core subnet would carry 50 percent of the traffic load, or 96 Mbps of traffic. Given this amount of traffic, the performance of the distribution switch must be capable of switching 187,000 packets per second. 

The Layer 3 module of the distribution-layer switch will be responsible for routing the remote traffic to the core. Therefore, a switch must be chosen that will support this amount of traffic. 

This topology presents no redundancy between the end user and the core. If the link between an access switch and distribution device fails, 100 users lose connectivity. If the distribution device fails, the whole building is disconnected from the network. One solution is to add a second distribution switch with backup links to each access switch.