Network Design, Switches and vLANs

Abstract

This chapter discusses methods that can be used to design a network, in its local connection, and in its connection to wider networks (including the Internet). A major problem in the past has been security and contention on Ethernet network. Switching overcomes both these problems as it can create vLANs (Virtual LANs) which allow nodes to be isolated from each other, even though they connect to the same device. Switches avoid the problem of contention on a network, as they take frames from one interface, and forward it to another. This can take place at either Layer 2 (Data Link layer) or Layer 3 (Network Layer). Routers implement Layer 3 switching to route data packets and switches (Layer 2 switches) use Layer 2 switching to forward frames. In Layer 3 switching the router examines the network address, and decides on whether it should forward it to another port, whereas with Layer 2 the switch examines the MAC address of the data frame (it thus ignores any network layer information). A Layer 2 switch must build and maintain a switching table that keeps track of the MAC addresses that belong to each port or interface. As with the ARP protocol, the Layer 2 switch sends out a broadcast on all of its ports if it does not know where to send a data frame. When the addressed node detects its address, it sends back an ARP reply, and the switch adds the new address to its switching table. The MAC address itself is a 48-bit address, made up of the manufacturing (MFG) code and the unique identifier. Organizations have no control over MAC addresses, and they have a flat addressing structure, whereas Layer 3 address are typically organized in a hierarchical way, typically using IP, IPX or AppleTalk addresses.