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Default Ethernet and Fast Ethernet Configuration
The Catalyst enterprise LAN switches solve congestion problems 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, an option for each port on the switches (Gigabit Ethernet ports only support
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 attached to the hub. If two stations establish a session that uses a
significant level of bandwidth, the network performance of all other stations 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 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 not yet 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, 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 Configuration
Table 4-1
Table 4-1
Feature
Port enable state
Port name
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
4-2
lists the Ethernet and Fast Ethernet default configuration.
Ethernet and Fast Ethernet Default Configuration
Default Value
All ports are enabled
None
Chapter 4
Configuring Ethernet and Fast Ethernet Switching
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