Cisco WS-C2980G-A Software Guide - Page 58

Default Ethernet and Fast Ethernet Configurations Chapter 4 Configuring Ethernet and Fast Ethernet Switching The Catalyst enterprise LAN switches solve congestion problems that are caused by high-bandwidth devices and a large number of users by assigning each device (for example, a server) to its own 10-, 100-, or 1000-Mbps segment. Because each Ethernet port on the switch represents a separate Ethernet segment, servers in a properly configured switched environment achieve full access to the bandwidth. Because the major bottleneck in Ethernet networks is usually due to collisions, an effective solution is full-duplex communication, which is an option for each port on the switches (Gigabit Ethernet ports support only full duplex). Normally, Ethernet operates in half-duplex mode, which means that stations can either receive or transmit. In full-duplex mode, two stations can transmit and receive at the same time. When packets can flow in both directions simultaneously, effective Ethernet bandwidth for Ethernet ports is 20 Mbps. For Fast Ethernet ports, it is 200 Mbps, and for Gigabit Ethernet ports, it is 2 Gbps. Switching Frames Between Segments Each Ethernet port on the switch can connect to a single workstation or server, or to a hub through which workstations or servers connect to the network. Ports on a typical Ethernet hub all connect to a common backplane within the hub, and the bandwidth of the network is shared by all devices that are attached to the hub. If two stations establish a session that uses a significant level of bandwidth, the network performance of all other stations that are attached to the hub is degraded. To reduce degradation, the Catalyst enterprise LAN switches treat each port as an individual segment. When stations on different ports need to communicate, the switch forwards frames from one port to the other at wire speed to ensure that each session receives the full bandwidth that is available. To switch frames between ports efficiently, the switch maintains an address table. When a frame enters the switch, it associates the Media Access Control (MAC) address of the sending station with the port on which it was received. Building the Address Table The switch builds the address table by using the source address of the frames received. When the switch receives a frame for a destination address that is not listed in its address table, it floods the frame to all ports of the same virtual LAN (VLAN) except the port that received the frame. When the destination station replies, the switch adds its relevant source address and port ID to the address table. The switch then forwards subsequent frames to a single port without flooding to all ports. The address table can store at least 16,000 address entries without flooding any entries. The switch uses an aging mechanism, which is defined by a configurable aging timer, so if an address remains inactive for a specified number of seconds, it is removed from the address table. Default Ethernet and Fast Ethernet Configurations Table 4-1 lists the Ethernet and Fast Ethernet default configurations. Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 4-2 78-15486-01