Wednesday, September 21, 2016

Cisco® Learning Path

Cisco® > Associate Level Certifications > Network > Routing & Switching> Switching Protocols > Spanning Tree Protocol (STP)> Introduction to the Logical loop free topology

Introduction to the Spanning Tree Protocol (STP)

The spanning tree protocol is a Layer 2 protocol that runs on bridges and switches. The main purpose of STP is to ensure that no loops are created when there are redundant paths available. Spanning Tree Protocol implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces.  STP can be used efficiently where redundant links are to be used avoiding loops. Without STP, a failure in the primary link can result in a loop.
Switching Loop in a non-Spanning Tree Protocol Environment
Any Layer 2 traffic can be categorized into Unicast (one-to-one), multicast (one-to-many) & broadcast (one-to-all). In case of multicast and broadcast environments, switches need to forward the packet to all ports (known as flooding) except the source port to make sure that the Ethernet frames reaches the destination.



Workstation III broadcasts a frame destined for Web Server. This frame reaches Switch III which forwards the frame to all the ports except the source port or the port at which the frame has been received. . Both, Switch II & Switch I will receive the frame. These switches will then attempt to search the destination MAC address in their MAC address table. If these switches aren’t able to look up the destination MAC address, they will again broadcast the frame to all the ports except the port at which the fame was received.  This might cause the Ethernet frame to reach Switch III which results into a switching loop.  This might result in an increase in the network overhead and affect network performance.