18-2
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
OL_28731-01
Chapter 18 Configuring Layer 2 Ethernet Interfaces
About Layer 2 Ethernet Switching
Layer 2 Ethernet Switching
Catalyst 4500 series switches support simultaneous, parallel connections between Layer 2 Ethernet
segments. Switched connections between Ethernet segments last only for the duration of the packet. New
connections can be made between different segments for successive packets.
Note With Cisco IOS Release 12.1(13)EW, the Catalyst 4500 series switches can handle packets of
1600 bytes, rather than treat them as “oversized” and discard them. This size is larger than the usual
IEEE Ethernet Maximum Transmission Unit (MTU) (1518 bytes) and 802.1q MTU (1522 bytes). The
ability to handle larger packets is required to support two nested 802.1q headers and Multiprotocol Label
Switching (MPLS) on a network.
The Catalyst 4500 series solves 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 interface on the switch represents a separate Ethernet segment, servers
in a properly configured switched environment achieve full access to the bandwidth.
Because collisions are a major bottleneck in Ethernet networks, an effective solution is full-duplex
communication. Normally, Ethernet operates in half-duplex mode, which means that stations can either
receive or transmit. In full-duplex mode, two devices can transmit and receive at the same time. When
packets can flow in both directions simultaneously, effective Ethernet bandwidth doubles to 20 Mbps for
10-Mbps interfaces and to 200 Mbps for Fast Ethernet interfaces. Gigabit Ethernet interfaces on the
Catalyst 4500 series switch are full-duplex mode only, providing 2-Gbps effective bandwidth.
Switching Frames Between Segments
Each Ethernet interface on a Catalyst 4500 series switch can connect to a single workstation or server,
or to a hub through which workstations or servers connect to the network.
On a typical Ethernet hub, all ports 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 devices 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 switch treats each interface as an individual segment. When stations on
different interfaces need to communicate, the switch forwards frames from one interface to the other at
wire speed to ensure that each session receives full bandwidth.
To switch frames between interfaces efficiently, the switch maintains an address table. When a frame
enters the switch, it associates the MAC address of the sending station with the interface on which it was
received.
Building the MAC Address Table
The Catalyst 4500 series builds the MAC address table by using the source address of the frames
received. When the switch receives a frame for a destination address not listed in its MAC address table,
it floods the frame to all interfaces of the same VLAN except the interface that received the frame. When
the destination device replies, the switch adds its relevant source address and interface ID to the address
table. The switch then forwards subsequent frames to a single interface without flooding to all interfaces.
The address table can store at least 32,000 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.
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