Network Concepts and Spanning Tree Protocol

Importance of Avoiding Loops

• MAC Address Level
  • No mechanism for counting frames on a network.
  • Creates a risk of loops, which can cause frames to circle endlessly.
• Spanning Tree Protocol (STP)
  • Essential for preventing loops in a switch network.

Switch Traffic Forwarding

• Unicast Packets
  • Have a single destination.
• Broadcasts and Multicasts
  • Sent to multiple devices simultaneously.
  • A single frame can exit every interface on a switch.

Time-to-Live (TTL) in IP

• Prevents indefinite looping of packets.
• Lacks equivalent in switch network frames.

Network Topology and Loops

• Loop Creation
  • Accidental extra connections can create loops.
  • Additional traffic can overwhelm switches, halting the network.
• Spanning Tree Protocol Activation
  • Activated by network administrators to prevent such loops.

Spanning Tree Protocol (STP) Functionality

• Bridge Protocol Data Unit (BPDU)
  • Used by STP to communicate configuration details between switches.
  • Default: Sent every 2 seconds to update network topology.
• Root Bridge Concept
  • Elected during network startup.
  • Key for communication pathways.
  • Manual configuration through lowest bridge ID.
  • Lowest MAC address chosen if IDs are identical.

Port States in STP

• Blocking/Discarding State
  • Prevents loops by blocking traffic on specific ports.
• Listening Mode
  • Listens for other switches to set configurations accordingly.
• Learning Mode
  • Updates MAC address table before enabling interfaces.
• Forwarding and Disabled Modes
  • Forwarding: Active traffic passing.
  • Disabled: Administrator turns off the port.

VLAN Configuration Issues

• VLAN Association
  • Each switch interface tied to a VLAN.
  • Misconfigured VLAN can prevent device communication.
• Access Ports
  • Single VLAN for single device.
  • Common issue in large networks but easily resolvable.

Troubleshooting Network Issues

• Access Control Lists (ACLs)
  • Similar to firewall rules, stopping evaluation after a match.
  • More granular controls should be prioritized on the list.
• Best Practices for ACLs
  • Disable ACLs before changes to avoid losing access.
  • Default action usually denies all traffic not specified.