Connect & Configure Cisco Networking Devices: Part 2

Table of Contents

1. Main Menu
2. Part 2: Basic Switch Operation and Configuration
3. 2.1: LAN Switching
4. LAN Switching Continued
5. 2.2: Basic Configuration Commands
6. 2.2.1: Privilege Exec Mode Commands
7. 2.2.1: Global Configuration Mode Commands
8. 2.2.3: Line Mode Commands
9. 2.3: Displaying the contents of the MAC Address Table
10. 2.4: Switch Interfaces
11. Switch Interface Configuration Continued
12. 2.5: Assigning an IP Address & Default Gateway to a 2950 Switch
13. 2.6: Configuring Port Security
14. Video: Basic Switch and Port Security Configuration
15. 2.7: The Spanning Tree Protocol (STP)
16. 2.7.2: STP Port States
17. 2.7.3: STP Configuration
18. Review Questions for Part 2

2.7.2: STP Port States

Both Path Cost and Bridge Priority can be modified on Cisco devices

The STP Root Bridge is the Bridge with the Lowest Bridge ID. This includes the Priority and Internal MAC Address of the Bridge. STP will place all ports into one of five main states:

• Blocking - This is the initial state. All ports are put in a blocked state to prevent bridging loops.

• Listening - The ports Listen for incoming BPDU messages to ensure a loop free topology, an interface in this state does NOT forward traffic or learn MAC address information

• Learning - The Port listens for incoming BPDU messages to ensure a loop free topology, an interface in this state learns MAC addresses but does NOT forward traffic

• Forwarding A port can send and receive data in this state. Before placing a port in forwarding state, STP ensures that there are no redundant paths or loops.

• Disabled - This is the state when the switch port is disabled. A switch port may be disabled due to administrative reasons or due to switch specific problems.

While the bridge is broadcasting BPDU packets, it is also listening for BPDU packets from other bridges. The bridge compares each message that it hears on the port, as well as the message that it would send on that port, and evaluates them to determine which one it will keep. The message that the bridge decides to keep will designate who the bridge thinks is the root bridge. If a bridge hears about another bridge that is better than it, it stops telling everybody else that it is the root and starts telling everybody else about the better bridge. Eventually, everybody agrees on who is the best bridge, and that bridge becomes the root.

Finally, each bridge deactivates ports in such a manner that there is only one path from any given point on the network to the root. All loops have now been eliminated.

Convergence will occur when all the Switch and Bridge ports have transitioned to either the forwarding or blocking state.

Whenever there is a topology change, each Switch / Bridge needs to run STP, which can severely disrupt user traffic.