In the initial stage, the switch does not know any MAC addresses at all.Let’s assume that the switch receives for first time a frame from A destined to B. What happens though when the switch receives a frame with a destination MAC address and it cannot find the same in the switch’s lookup table.As part of the learning process, a switch will flood the single frame out all of its other ports.This process is called unknown unicast flooding. The ARP response also updates the switch’s lookup table. The ARP Response contains the MAC address of the intended target. ![]() The inbound ARP contains the sender’s MAC address, and the switch adds that address to its lookup table. Most often, the initial communication to a switch is an Address Resolution Protocol (ARP) request. When a frame arrives headed for that address, the switch can compare the aging timer for both entries and select the most recent if the entry has not already timed out. In that case, the address would appear in both places in the lookup tables. The aging timer also helps when a device moves to another switch port. The timer lets the switch get rid of old entries to keep the lookup process short and fast. By default, many vendors set this time to hold an address entry to 300 seconds (5 minutes). The switch learning process works like this:ġ.As a PC or other networked device sends a frame to another device through the switch, the switch captures the source MAC address of the frame and the interface that received it.Ģ.The switch confirms or adds the MAC address and the port to the lookup table.Ī switch also keeps a timer for each of the MAC address entries in its lookup table. ![]() This learning capability makes switches easy to use on a network. Switches Learn, Filter, Forward and Flood Ethernet frames.Ī switch begins learning the local MAC addresses as soon as it is connected to other devices or to a network. A switch operates on layer 2 of the OSI model which means that it processes frames and forwards them based on MAC addresses.
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February 2023
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