Spanning Tree Protocol (STP) - Cisco Systems

       The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links.
       Spanning Tree Protocol (STP) is standardized as IEEE 802.1D. As the name suggests, it creates a spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes.

Use the Diagram of the Network before you troubleshoot a bridging loop... CLICK>>>>

You need to know these items, at minimum:

• The topology of the bridge network
• The location of the root bridge
• The location of the blocked ports and the redundant links

This knowledge is essential for at least these two reasons:

• In order to know what to fix in the network, you need to know how the network looks when it works correctly.
• Most of the troubleshooting steps simply use show commands to try to identify error conditions. Knowledge of the network helps you focus on the critical ports on the key devices.

The following algorithm is used to determine the root port or designated port (which is your best path to the root):

1. Lowest root bridge id
2. Lowest root path cost
3. Lowest sender bridge id
4. Lowest sender port number

RSTP Operation RSTP adds new bridge port roles in order to speed convergence following a link failure.
RSTP bridge port roles:
Root - A forwarding port that is the best port from Nonroot-bridge to Rootbridge
Designated - A forwarding port for every LAN segment
Alternate - An alternate path to the root bridge. This path is different than using the root port.
Backup - A backup/redundant path to a segment where another bridge port already connects.
Disabled - Not strictly part of STP, a network administrator can manually disable a port

 

The previous diagram illustrates the Spanning Tree Physical Cable/Logical Diagram 

 

 The previous diagram illustrates the Spanning Tree (STP) scenario

Useful Commands
Cisco IOS Software Commands 

• show interfaces
• show spanning-tree
• show bridge
• show processes cpu
• debug spanning-tree
• logging buffered

References: 

• http://www.cisco.com/…/technologies_tech_note09186a008009482f.shtml 
• http://www.cisco.com/…/guide/stp_enha.html#wp1033403
• http://www.cisco.com/…/technologies_tech_note09186a00800ae96b.shtml

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