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Tuesday, April 04, 2006


Lan Switches

So, what are LAN switches? Switches are essentiallY mult-port bridges. Switches operate on the same basic principle as bridges. The difference is that essentially each host is often connected directly to a port on the switch, effectively resulting in each host having its own dedicated segment (microsegmentation). By examining MAC addresses the switch learns where hosts are located and forwards frames only to the necessary port. Because the decision to forward packets is based on layer 2 addresses, these types of switches are often called frame switches. Note: Some vendors also sell LAN switches that incorporate functions that operate on layer 3 information. Such switches are often referred to as multi-layer switches.

The benefits of switches are enormous. With Full-Duplex ethernet support, collisions can be virtually eliminated. Each host on the switch essentially has access to the full amount of available bandwidth.

There are two primary of Lan Switching modes, Store & Forward and Cut Through.

Store & Forward

This is the mode used by Catalyst 5000 series switches. In this mode an entire frame is read into a memory buffer on the switch. The frame is then analyzed for errors (CRC computation). If the frame is good, the switch consults its table of known MAC addresses and forwards the frame to the appropriate port. This method has the benefit of having each frame checked for errors and discarded if mal-formed. However, because it must read the entire frame into memory and peform the CRC, there is a higher degree of latency when compared to other methods.

Cut Through

This is an option in some EtherSwitch models. In Cut-Through switching, only the destination MAC address is read into memory. This is done simply to determine to which port to forward the frame. Once the destination port is known the switch immediatly begins forward the frame to that port. It does not do any error checking. The benefit of this method of switching is reduced latency but at the cost of potentially sending unwanted, mal-formed frames to host computers. Some cut-through switches attempt to reduce problems by filtering out collision fragments. Collision fragments are less than 64 bytes, so the switch reads 64bytes before beginning to forward the frame. Cisco refers to the standard cut-through switch as Fast Forward and those that filter collision fragments as Fragment Free.

Virtual LANs

Another great benefit of modern switches is a capability to created virtual LANs (VLANs). After a VLAN is established on a switch, frames (broadcast, multicast, or unicast) will only be forwarded to that particular VLAN. VLANs become particularly beneficial in their capability to span switches. No longer is the physical location of a host the determining factor in which LAN it belongs. This is accomplished by inserting a VLAN ID into the frame that identifies to which VLAN the frame belongs. This is used only in the switch framework and is removed before the final switch in a network forwards the frame to the destination port.


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