Functions of Layer-2 Switching
There are three distinct functions performed at OSI layer-2 (the Data-Link layer) namely Learning, Forwarding & Filtering and Loop Avoidence. These functions are explained in following sections;
1) Learning
When switch starts, the MAC address table has no entry. When a node transmits data on its wire the MAC address of the node is learned by Switch Port connected to that node. In this way all the MAC addresses are learned by respective ports and these entries remain in the cache for a specific time. If during this specific time no new frame arrives from a node MAC address entry for that node is dropped from cache.
When switch starts, the MAC address table has no entry. When a node transmits data on its wire the MAC address of the node is learned by Switch Port connected to that node. In this way all the MAC addresses are learned by respective ports and these entries remain in the cache for a specific time. If during this specific time no new frame arrives from a node MAC address entry for that node is dropped from cache.
2) Forwarding & Filtering
When a MAC address for a port is learnt, packets addressed to that MAC address are forwarded only to the port associated with it, using one of the Switching Methods (i.e Store & Forward, Cut Through & Fragment Free).
3) Loop Avoidance
Switches and Bridges use Spanning Tree Protocol (STP), specified by IEEE 802.1d, or one of its variants to prevent loops. Loops can be caused by multiple connections between switches, established for redundancy.
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