Spanning Tree the problem and the solution

Spanning tree protocol or STP is used to provide redundant links in the network while preventing the danger of bridging loops.

Our network below has two switches connected to each other by two FastEthernet links for redundancy. In such setups STP is essential to avoid bridging loops.

The Problem:How bridging loops happen ? (STP is disabled)

  1. PC1 sends an ARP request to find the MAC address of PC2.
  2. Switch1 receives the ARP (Broadcast) and forwards it to all ports except the receiving one.
  3. Switch2 receives the ARP request and forwards it to all ports except the receiving one including the redundant link.
  4. Switch1 receives the frame again from Switch2 over the redundant link between them and send it again to switch2 and a loop forms.
  5. If this broadcast storm is strong enough or amplified by other switches its capable of bringing the whole network down.

When loops occur some switches see workstations on both sides which confuses the switch allows duplicate frames to be forwarded.

The Solution:How does STP solve the problem ?

STP was developed to allow using redundant links in the bridging network while maintaining a loop free network. STP builds a tree (loop free) topology for the network by exchanging special messages called BPDUs between bridges, blocking and activating redundant links dynamically.

If a topology change occurs, the tree topology is recalculated and the normal operation of the network is maintained by blocking and activating links again.

In upcoming posts we are going to explore STP operation in more details.

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