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Switches learning MAC addresses

November 24, 2012

The filter-versus-forward decision works best when the switches know where all the MAC addresses are in the network. Switches dynamically learn the MAC addresses in the network to build its MAC address table. With a full and accurate MAC address table. The switch can make accurate forwarding decisions.

Forwarding Unknown Unicasts and Broadcasts


Bridges forward LAN broadcast frames, and unknown Unicast frames, out all ports, All devices on the same LAN receive LAN broadcasts. The switch forward broadcasts out all ports, except the one on which the broadcast was received. Switches forward unknown Unicast frames, which are frames whose destination MAC addresses are not yet in the bridging table, out all ports as well. The switch floods the frame with the hope that the unknown device will be on some other Ethernet segment, it will reply, and the switch will build a correct entry in the address table.


Explaining Spanning-Tree Protocol


Spanning-Tree Protocol is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between two stations. To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all switches in an extended network.


The current 802.1d Spanning Tree Protocol (STP) standard was designed at a time where recovering connectivity after an outage within a minute or so was considered adequate performance. With the advent of Layer 3 (L3) switching in LAN environments, bridging now competes with routed solutions where protocols such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) are able to provide an alternate path in less time.


Cisco enhanced the original 802.1d specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of a bridged network. The drawback is that these mechanisms are proprietary and need additional configuration. Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1d standard more than left unchanged so users familiar with 802.1d can rapidly configure the new protocol comfortably.


Spanning-Tree Operation


Spanning-Tree operation begins with the election of a root bridge. There is only one bridge designated as the root bridge in a given network. On the rot bridge, all its ports are designated ports. Designated ports are normally in the forwarding state. When in the forwarding state, a port can send and receive traffic. In the example, Switch X is elected as the root bridge.


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  1. eshu permalink

    superb site this is…………….useful to all dept students………..

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