Purpose of STP

Module 5: STP Concepts

5.1 Purpose of STP

• Redundancy in Layer 2 Switched Networks

  • Redundancy is crucial in hierarchical design to eliminate single points of failure.
  • Requires both physical and logical redundancy.
  • Alternate physical paths enable access to network resources despite disruptions.
  • Redundant paths can cause physical and logical Layer 2 loops.
  • Ethernet LANs need a loop-free topology with only one path between devices.
  • Loops cause continuous frame propagation until a link breaks.

• Spanning Tree Protocol (STP)

  • STP is a loop-prevention network protocol that creates a loop-free Layer 2 topology while allowing redundancy.
  • Logically blocks physical loops to prevent frames from endlessly circling.

• STP Recalculation

  • STP compensates for network failures by recalculating and opening previously blocked ports.

• Issues with Redundant Switch Links

  • Path redundancy eliminates single points of failure.
  • Without spanning tree, Layer 2 loops occur when multiple paths exist between devices.
  • Layer 2 loops lead to MAC address table instability, link saturation, and high CPU utilization, rendering the network unusable.
  • Ethernet lacks a mechanism to eliminate endlessly looping frames, unlike IPv4 and IPv6, which use TTL and Hop Limit fields, respectively.
  • STP was developed as a loop prevention mechanism for Layer 2 Ethernet.

• Layer 2 Loops

  • Without STP, broadcast, multicast, and unknown unicast frames loop endlessly, potentially crashing the network.
  • Loops cause continuous MAC address table changes, leading to MAC database instability and high CPU utilization, preventing frame forwarding.
  • An unknown unicast frame occurs when a switch lacks the destination MAC address in its table, forcing it to forward the frame out all ports except the ingress port.

• Broadcast Storm

  • A broadcast storm is an abnormally high number of broadcasts overwhelming the network.
  • It can disable a network quickly by overwhelming switches and end devices, caused by hardware issues like faulty NICs or Layer 2 loops.
  • Layer 2 broadcasts like ARP Requests are common; multicasts are typically forwarded similarly.
  • IPv6 uses Layer 2 multicasts for ICMPv6 Neighbor Discovery instead of broadcasts.
  • A host in a Layer 2 loop is inaccessible, and the switch cannot forward unicast frames due to constant MAC address table changes.
  • Spanning tree is enabled by default on Cisco switches to prevent Layer 2 loops.

• The Spanning Tree Algorithm

  • STP is based on Radia Perlman's spanning tree algorithm (STA), published in 1985.
  • STA creates a loop-free topology by selecting a single root bridge and determining the least-cost path for other switches.
  • STP blocks redundant paths using