STP

📘 COMPREHENSIVE NOTES ON SPANNING-TREE PROTOCOL (STP)

🧩 I. STP FUNDAMENTALS & NECESSITY

A. Purpose of STP

• Prevents Layer 2 loops when redundant links exist between switches.
• Redundant links ensure redundancy but can cause broadcast storms without STP.
• STP ensures loop-free topology by blocking redundant paths dynamically.

B. The Loop Problem

• In a switched network, broadcasts (e.g., ARP) flood all ports except the ingress.
• With redundant links, broadcasts can loop indefinitely, creating:
  • Continuous MAC table updates (CAM flapping)
  • High CPU utilization
  • Network congestion or outage

C. STP Mechanism

• STP detects redundant paths using BPDUs (Bridge Protocol Data Units).
• It blocks one or more redundant links to form a logical loop-free tree.
• Uses port roles and port states to control data flow.

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🏛️ II. CORE STP CONCEPTS & ELECTION PROCESS

A. Root Bridge Election

1. Role of Root Bridge:
   • Central reference point of the STP topology (“King of the network”).
   • Determines which ports on other switches forward or block.
2. Per VLAN Election:
   • Only one Root Bridge per VLAN.
   • PVST (Per-VLAN Spanning Tree) runs separate instances per VLAN.
3. Root Bridge Selection Criteria:
   1. Lowest Bridge Priority
   2. If tied, lowest MAC Address
4. Priority Details:
   • Default priority: 32768
   • Must be multiple of 4096
   • If priorities are equal → Lowest MAC wins

B. Bridge ID

• Bridge ID = System Priority + MAC Address
• Used in BPDU comparison; lower Bridge ID = higher priority.

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⚙️ III. STP COST & PORT SELECTION CRITERIA

A. STP Cost

• STP assigns a cost value to each interface speed.

Interface Type	Default Cost (PVST/Rapid PVST)
10 Mbps (Ethernet)	100
100 Mbps (Fast Ethernet)	19
1 Gbps	4
10 Gbps	2

• Root Bridge advertises cost = 0
• Total Path Cost = Local Port Cost + Received BPDU Cost

B. Port Roles

Port Role	Description	Notes
Root Port (RP)	Best path from a non-root bridge to the Root Bridge	1 per switch/VLAN
Designated Port (DP)	Best port on a link to forward frames toward Root	1 per segment
Non-Designated / Blocked	Prevents loops by staying blocked	Backup path

C. Port Selection Criteria (Four-Way Tie Breaker)

1. Lowest Path Cost to Root Bridge
2. Lowest Sender Bridge ID
3. Lowest Port Priority (Default = 128)
4. Lowest Port Number

Key Rules:

• All Root Bridge ports are Designated & Forwarding.
• Every non-root switch must have one Root Port.
• On shared links, the switch with the lowest cost to the Root wins the Designated Port.

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🔄 IV. STP MODES & PORT STATES

A. Per-VLAN Spanning Tree (PVST)

Port State	Function	Default Timer
Disabled	Admin down	N/A
Blocking	Receives BPDUs only	N/A
Listening	Sends/receives BPDUs, no MAC learning	15s
Learning	Learns MACs, no frame forwarding	15s
Forwarding	Full data forwarding	N/A

Convergence Time:
≈ 32–34 seconds (15s Listening + 15s Learning + 2s Hello)

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B. Rapid Per-VLAN STP (Rapid PVST / RSTP)

• Faster convergence (~6s or less).
• TCNs (Topology Change Notifications) are sent bidirectionally.
• Port States:
  • Discarding (combines Disabled, Blocking, Listening)
  • Learning
  • Forwarding

Port Roles:

• Root Port (RP)
• Designated Port (DP)
• Alternate Port: Backup for Root Port
• Backup Port: Backup for Designated Port

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C. Multiple Spanning Tree (MST)

• IEEE 802.1s standard (vendor-neutral).
• Groups multiple VLANs into a single STP instance to reduce CPU load.

Key Characteristics:

• Instance-based, not per VLAN.
• Configured per instance:

spanning-tree mst instance <id> priority <value>

• Default Cost (Fast Ethernet) = 200,000

• Boundary Port: Interface connecting MST to PVST domain.

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🛠️ V. STP CONFIGURATION & ENHANCEMENTS

A. Influencing Root Bridge Selection

1. Set Manual Priority

spanning-tree vlan <vlan-id> priority <value>

1. Root Primary/Secondary Shortcuts

spanning-tree vlan <vlan-id> root primary
spanning-tree vlan <vlan-id> root secondary

• root primary sets priority to 24576 (if all others are default)

B. Influencing Port Selection

1. Change STP Cost (local influence)

spanning-tree cost <value>

1. Change Port Priority (upstream influence)

spanning-tree port-priority <value>

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C. STP Fast Convergence Features

1. PortFast

• Used for edge/access ports connected to end hosts.
• Immediately transitions to Forwarding state (skips Listening/Learning).
• Must not be used on switch-to-switch links.

Command:spanning-tree portfast

2. UplinkFast

• Enables fast failover for uplink/trunk ports.
• Works only for direct link failures.
• Adds cost +3000 and raises bridge priority to 49152 (in PVST).
• Not needed in Rapid PVST, as RSTP has it built-in.

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⚖️ VI. LOAD BALANCING USING STP

• Achieved by assigning different Root Bridges for different VLAN ranges.
• Example:
  • Switch1 → Root for VLANs 1–10
  • Switch2 → Root for VLANs 11–4094
• Ensures VLAN-based traffic load sharing across redundant links.

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🎯 KEY INTERVIEW REVISION POINTS

Topic	Key Focus
Root Bridge Election	Priority (lowest wins) + MAC tie-breaker
STP Costs	Lower = preferred path
Port Roles	Root, Designated, Alternate, Backup
STP Timers	Hello (2s), Forward Delay (15s), Max Age (20s)
Rapid PVST Advantage	<6s convergence
MST Benefit	VLAN grouping = CPU optimization
PortFast Use Case	Host connections only
Load Balancing	Different roots for different VLANs

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🧠 QUICK MEMORY TIPS

• STP = Loop Prevention @ Layer 2
• BPDU = Heartbeat of STP
• Root Bridge = Boss; RPs report to it
• PVST → Rapid PVST → MST = Cisco’s STP evolution
• Fast Convergence = PortFast + UplinkFast (or RSTP)

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