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Spanning Tree Protocol prevents looping frames in a LAN with redundant links.
Why we need Spanning Tree Protocol (IEEE 802.1D)?
To prevent
· Broadcast storms
· MAC table instability
· Multiple frame transmission
When broadcast storms happen?
Broadcast storms occur when any form of Ethernet frames – broadcast, multicast, or unknown destination unicast frames loop around a LAN endlessly.
For example, if 3 switches Sw1, Sw2, and Sw3 are connected with each other. When a host named Tom connected with Sw1 sends a broadcast frame, according to the switch logic, Sw1 floods broadcast frames out all interfaces in the same VLAN except the interface in which the frame arrived. Sw1 forwards the frame to Sw2, Sw2 forwards the frame to Sw3 and Sw3 forwards it back to Sw1 again. This leads to broadcast storm.
Also, note that Sw1 forwards the frame not only to Sw2 but also to Sw3. The above event also happens in the opposite direction that is, Sw1 -------------Sw3-----------SW2------------Sw1-----------Sw2 & Sw3 – continuously growing loop.
When broadcast storms occur, frames keep looping until someone shuts down an interface, reloads a switch or does something else to break the loop.
MAC table instability
Because of the broadcast storm, switches’ MAC address table keeps changing as the frames with the same source MAC enter on different ports. As you know, Switches update the MAC address table according to the incoming switch port of any frame.
Multiple frame transmission
Switches flood frames sent to unknown unicast MAC addresses. Because of redundant links which causes looping the unknown destination host connected to a switch receive multiple copies of the frame. This may result in application failure by confusing the destination host.What is NAT in Networking?
What is Router on a stick?
Syslog Severity Levels – Explained.
VLAN Trunking Protocols – 802.1q & ISL.
How a MAC address is universally unique?
What is Syslog – System Message logging?
Media Access Methods – CSMA/CD, CSMA/CA, Token passing & Polling, Explained?
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