Introduction to STP

Posted on July 23, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , , |

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Physical Segment, Collision Domain and Redundent Topology

Redundent Topology Caused three problems:

  • Broadcast Storm
  • Multiple Frame Copies
  • MAC database Instability

As the solution, Spanning Tree Protocol (STP) creates a loop-free topology by putting certain ports in blocking state. STP is enabled by default on Cisco switches.

There are three basic rules for spanning tree:

  1. There’s one root bridge per network.
  2. There’s one root port per non-root bridge.
  3. There’s one designated port per one segment.

Root bridge is the bridge with the best bridge ID. The root bridge is the focal point in the network. Important decisions such as which port is to be blocked and which port is to be put in forwarding mode – are made from the prespective of the root bridge.

The root bridge is elected with Bridge ID. Bridge ID have two parts: bridge priority (32,768 by default on all Cisco switches) and MAC address. If the bridge priority is the same, the bridge with lowest MAC address will be elected as the root bridge.

The Root Port is the link directly connected to the root bridge, or the shortest path to the root bridge. If more than one link connects to the root bridge, then the lowest-cost port becomes the root port.

A designated port is one that has been determined as having the lowest cost. A designated port will be marked as a forwarding port.

A nondesignated port is one with a higher cost than the designated port. Nondesignated ports are put in blocking mode — they are not forwarding ports.

The STP decides which port is root port and which port is desigated port and which port to block by the concept of the cost. Cost is associated with bandwidth. The faster the path to the root bridge, lower the cost.

The root bridge send out Bridge Protocol Data Unites (BPDU‘s) every two seconds. If those BPDU are not received, the non-designated bridges know that the root bridge is down. These non-designated bridges then transition their port from:

  • Blocking to
  • Listening to
  • Learning to
  • Forwarding

Blocking A locked port won’t forward frames; it just listens to BPDUs. All ports are in blocking state by default when the seitch is powered up.

Listening The port listens to BPDUs to make sure no loops occur on the network before passing data frames. A port in listening state prepares to forward data frames without populating the MAC address table.

Learning The switch port listens to BPDUs and learns all the paths in the switched network. A port in learning state populates the MAC address table but doesn’t forward data frames. Forward delay means the time it takes to transition a port from listening to learning mode, which is set to 15 seconds by default.

Forwarding The port sends and receives all data frames on the bridged port. If the port is still a designated or root port at the end of the learning state, it enters the forwarding state.

Spanning Tree Convergence is reached when all ports have transition into either blocking state or forwarding state. No data will be forwarded until convergence is complete.

ICND1 and ICND2 break down


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