Understanding Layer 2 Ethernet Switching - Cisco 7604 Configuration Manual

Understanding How Layer 2 Switching Works

Understanding Layer 2 Ethernet Switching

These sections describe Layer 2 Ethernet switching:
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Layer 2 Ethernet Switching Overview
Cisco 7600 series routers 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.
Cisco 7600 series routers solve congestion problems caused by high-bandwidth devices and by 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 cause significant congestion 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, the effective Ethernet bandwidth doubles.
Switching Frames Between Segments
Each LAN port on a Cisco 7600 series router 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 router considers each LAN port to be an individual segment. When stations
connected to different LAN ports need to communicate, the router 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 router maintains an address table. When a frame
enters the router, it associates the MAC address of the sending network device with the LAN port on
which it was received.
Building the Address Table
Cisco 7600 series routers build the address table by using the source address of the frames received.
When the router 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 router adds its relevant source address and port ID to the address table. The router
then forwards subsequent frames to a single LAN port without flooding to all LAN ports.
The address table can store at least 32,000 address entries without flooding any entries. The router 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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
10-2
Layer 2 Ethernet Switching Overview, page 10-2
Switching Frames Between Segments, page 10-2
Building the Address Table, page 10-2
Chapter 10 Configuring LAN Ports for Layer 2 Switching
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