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Data Communication & networking

This blog is for undergraduate/graduate students who require some basic information about their subjects or any other topic related to data communication.

Monday, 28 March 2011

Layer 2 Switch

Layer 2 Switch:
Layer 2 switch is A network device that forwards traffic based on MAC layer (Ethernet or Token Ring) addresses. Layer 2 switches effectively provide the same functionality. They are similar to multiport bridges in that they learn and forward frames on each port. The major difference is the involvement of hardware that ensures that multiple switching paths inside the switch can be active at the same time. the layer 2 switches are some time called as switching hub.
The layer 2 switches have several advantages,
  • No change is required to the software and hardware device.
  • Each attached device has a dedicated capacity equal to the entire original LAN.
  • The layer 2 switch scales easily.
 There are two types of layer 2 switches.
  1. STORED AND FORWARD:  It accepts a frame on an I/P line, store it briefly and then routes it to an O/P line.
  2. CUT THROUGH SWITCH: It repeats the incoming frames on the O/P line as soon as layer 2 switch recognizes the destination address.
LIMITATIONS: 
As the number of devices in a building increase, layer 2 switch reveals some problems.
  1. Broadcast Overload  A set of devices &LAN connected by layer 2 switch are considered to have a flat address. The flat address means that all users share a common MAC broadcast address (In LANs some protocols ,e.g. ARP, work in broadcast manner).
  2. Lack of Multi links: A second problem related to layer 2 switch is that current standard for bridge protocols dictate that there be no closed loop in the network. that is, there can be be onle path between any two devices (Limits both performance and reliability).
SOLUTION: To overcome these problems, it seems logical to break up a large local network into a number of sub networks connected by routers. A MAC broadcast frame is then limited to only the device and switch contained in a single sub network.
Furthermore, IP based router employ sophisticated routing algorithms that allow use of multiple paths between sub networks going through different routers.

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