Understanding Vlan Trunks - Cisco 6500 Series Software Configuration Manual

Understanding How Layer 2 Switching Works
Layer 2 Ethernet Switching Overview
Catalyst 6500 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 the next packet.
Catalyst 6500 series 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
collision domain. Because each LAN port connects to a separate Ethernet collision domain, 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 stations 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 ports and to 200 Mbps for Fast Ethernet ports. Gigabit Ethernet ports on Catalyst 6500 series
switches are full duplex only, providing 2-Gbps effective bandwidth.
Switching Frames Between Segments
Each LAN port on a Catalyst 6500 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 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 switch considers each LAN port to be an individual segment. When stations
connected to different LAN ports need to communicate, the switch forwards frames from one LAN port
to the other at wire speed to ensure that each session receives full bandwidth.
To switch frames between LAN ports efficiently, the switch maintains an address table. When a frame
enters the switch, it associates the MAC address of the sending station with the LAN port on which it
was received.
Building the Address Table
Catalyst 6500 series switches build the 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 address table, it floods the
frame to all LAN ports of the same 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 LAN port without flooding to all LAN 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.

Understanding VLAN Trunks

These sections describe VLAN trunks on the Catalyst 6500 series switches:
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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
7-2
Trunking Overview, page 7-3
Encapsulation Types, page 7-4
Chapter 7 Configuring LAN Ports for Layer 2 Switching
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