Supporting Multiple Speeds on a Single Switch

Supporting Multiple Speeds on a Single Switch

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The switch can easily support both speeds in the single switch, mainly because the only difference between Ethernet and Fast Ethernet is in how the bits are transmitted over the cable, and at what speed. Because the headers and trailers are the same, and the addresses are the same, the switch still learns MAC addresses, makes forwarding and filtering decisions, and buffers frames, just like always.

The switch in Figure 6-12 has physical ports that only support 10 Mbps or 100 Mbps; this type of switch does not provide an easy migration path from 10 Mbps to 100 Mbps. For instance, imagine that you have 12 devices migrated to use Fast Ethernet, so you have plugged them all into the 12 Fast Ethernet ports on the switch. Now Fred gets a new 100-Mbps NIC, puts it in his computer, and wants to be moved to a 100-Mbps switch port. Uh oh. No more ports.

To solve problems like this one, the IEEE created a standard, called autonegotiation, that provides an easy migration path from 10 Mbps Ethernet to 100 Mbps Ethernet, or even to 1000 Mbps Ethernet (GigE). Vendors build switches with physical ports that can run at various speeds. Autonegotiation allows the switch and NIC on either end of the cable to automatically negotiate to determine the speed. The switch and computer also happen to negotiate whether to use full duplex or half duplex. Figure 6-13 shows the general idea.