Lecture 11: Spanning Tree Protocol (STP) Flashcards
(12 cards)
What is STP?
STP is a Layer 2 loop-prevention protocol that creates a loop-free logical topology while allowing physical redundancy.
What issues occur without STP?
Loops, broadcast storms, MAC table instability, high CPU usage, and more.
Who invented the STP algorithm and when?
Radia Perlman in 1985 while working at Digital Equipment Corporation.
How does STP create a loop-free topology?
- Elects a Root Bridge (central switch)
- Blocks redundant paths to prevent loops
- Creates one logical path from each switch to the root
- Recalculates paths and reactivates blocked ports if failure occurs
What are the 4 main steps in STP?
- Elect the Root Bridge
- Elect Root Ports (lowest-cost path to root)
- Elect Designated Ports (best path on a network segment)
- Elect Alternate (Blocked) Ports
What is a BPDU in STP?
Bridge Protocol Data Unit – control messages used for STP decisions.
How is the best path (root port) determined?
Based on path cost – lower cumulative link cost to the Root Bridge wins.
What are the STP variants and key differences?
STP – Classic version (802.1D)
PVST – Per-VLAN STP
Rapid PVST – Faster convergence (802.1w)
How to configure different STP modes?
spanning-tree mode stp (classic)
spanning-tree mode pvst
spanning-tree mode rapid-pvst
What does PortFast do?
It allows a port to enter forwarding state immediately, bypassing listening and learning states — useful for end devices.
What is BPDU Guard used for?
Protects the network by shutting down edge ports if they receive BPDUs. Not recommended between switches.
What are the key differences between STP and RSTP?
- STP has states: Blocking, Listening, Learning, Forwarding
- RSTP replaces Blocking/Listening with Discarding
- In RSTP, all switches send BPDUs every 2 sec
- RSTP has Backup and Alternate port roles
