Lecture Notes on Network Loop Prevention and Spanning Tree Protocol

Key Concepts

• MAC Address Level

  • No mechanism for counting frames on a network.
  • Importance of avoiding loops on a switch network.

• Switch Traffic Forwarding

  • Based on destination MAC address.
  • Unicast packets have single destinations.
  • Broadcasts and multicasts sent to multiple devices simultaneously.

Loop Prevention

• Importance of Spanning Tree Protocol (STP)
  • Prevents loops that cause traffic to circle indefinitely.
  • Switch networks can easily create loops with accidental extra connections.
  • Spanning Tree prevents network overload and halts.

Spanning Tree Protocol (STP)

• Bridge Protocol Data Unit (BPDU)

  • Used to identify switches and prevent loops.
  • Sent as MAC layer multicast to local broadcast domains every 2 seconds.
  • Missed updates indicate a topology change.

• Root Bridge Election

  • Elected when the network starts.
  • Chosen based on lowest Bridge ID (0 to 61,240) and MAC address.

• Network Configuration

  • Switches configure settings to connect to the root bridge.
  • Designated ports are active for traffic.
  • Blocked ports prevent loops.

Network Recovery and Configuration

• Handling Network Changes

  • STP reconfigures network if connectivity is lost.
  • Updates port states to maintain network communication.

• Port States in STP

  • Blocking/Discarding: Prevents traffic on certain interfaces.
  • Listening: Prepares for changes by observing network traffic.
  • Forwarding/Blocking: Enables or disables traffic based on configuration.
  • Disabled Mode: Manually turned off by admin.

VLAN and Configuration Issues

• VLAN Configuration

  • Interfaces associated with specific VLANs (e.g., 254, 100).
  • Access ports assigned specific VLANs.
  • Common issues arise from incorrect VLAN settings.

• Access Control Lists (ACLs)

  • Similar to firewall rules.
  • Granular rules should be prioritized at the top.
  • Changes to ACLs can affect network access.
  • Default behavior often denies all non-listed traffic.

Troubleshooting Steps

• Checking VLAN Assignments

  • Ensure correct VLAN ID for connected devices.

• Evaluating Access Control Lists

  • Check ACLs on switches and routers to ensure no blockages.
  • Disable ACL functionality before making changes to prevent loss of access.

• Network Efficiency

  • Proper ACL configuration improves network efficiency.