D-Link DWS-3227P - xStack Switch - Stackable Instrukcja Użytkownika Pobierz pdf (Strona 4)

Semaphore Technologies Pvt. Ltd. – (technolo[email protected]om)

Layer 2

A single LAN switch, operating at the MAC sub layer of the data link layer, may

interconnect a small number of devices in a home or office. This is a trivial case of

bridging, in which the switch learns the MAC address of each connected device.

Compared to shared-medium LANs, a switch using micro segmentation prevents

collisions on an Ethernet, and can provide effectively simultaneous paths among

multiple devices. Single switches also can provide extremely high performance in

specialized applications such as storage area networks

Switches may also interconnect using a spanning-tree protocol that allows the best

path to be found within the constraint that it is a tree. In contrast to routers, bridges

must have topologies with only one active path between two points. The older IEEE

802.1d spanning tree protocol could be quite slow, with forwarding stopping for 30-

90 seconds while the spanning tree recon verged. A Rapid Spanning Tree Protocol

was introduced as IEEE 802.1w, but the newest edition of IEEE 802.1d-2004, adopts

the 802.1w extensions as the base standard.

Once a Layer 2 switch learns the topology through a spanning tree protocol, it

forwards data link layer frames using some variant of bridging. There are four

forwarding methods a Layer 2 switch can use:

Store and forward

The switch buffers and, typically, performs a checksum on each frame before

forwarding it on.

Cut through

The switch reads only up to the frame's hardware address before starting to

forward it. There is no error checking with this method.

Fragment free

A method that attempts to retain the benefits of both "store and forward" and

"cut through". Fragment free checks the first 64 bytes of the frame, where

addressing information is stored. This way the frame will always reach its

intended destination. Error checking of the actual data in the packet is left for

the end device in Layer 3 or Layer 4 (OSI), typically a router.

Adaptive switching

A method of automatically switching between the other three modes.

Note that cut-through switches have to fall back to store and forward if the outgoing

port is busy at the time the packet arrives.

Note that these forwarding methods are not controlled by the user and are

configured only by the switch itself.

Layer 3

Router is a marketing term for a Layer 3 switch, typically a router optimized for

Ethernet interfaces. Like other switches, it connects devices to single ports for micro

segmentation. The ports normally operate in full duplex.

Switches, even primarily Layer 2 switches, can be aware of Layer 3 multicast and

increase efficiency by delivering the traffic of a multicast group only to ports where

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