STP & RSTP

🌐 Spanning Tree Protocol (STP) & Rapid Spanning Tree Protocol (RSTP) — Technical Notes

📌 Introduction: Why STP is Critical

• Ethernet LANs use redundant paths for fault tolerance.

• Layer 2 Loops can occur due to lack of TTL (Time-to-Live) in Ethernet frames.

• Loops cause:

  • Broadcast Storms (uncontrolled BUM traffic)

  • MAC Address Flapping (instability in switch MAC tables)

• STP (802.1D) and RSTP (802.1w) solve these by:

  • Creating a loop-free logical topology

  • Blocking redundant links

🔧 STP (Spanning Tree Protocol) - IEEE 802.1D / Cisco PVST+

🔸 STP Overview

• Default protocol on Cisco switches.

• PVST+: Cisco’s version — one STP instance per VLAN (Per-VLAN STP).

• Operates via BPDU (Bridge Protocol Data Unit) exchanges every 2 seconds.

🔹 STP Algorithm: 3-Step Process

1. Root Bridge Election

Switch with the lowest Bridge ID (BID) becomes Root Bridge.

BID = Bridge Priority (default: 32768 + VLAN ID) + MAC Address.

Commands:

spanning-tree vlan 10 priority 0
spanning-tree vlan 10 root primary

• Verify: show spanning-tree

2. Root Port Selection

• On non-root switches, select port with lowest path cost to the root.

• Tie-breakers (in order):

  • Lowest Root Path Cost

  • Lowest Neighbor BID

  • Lowest Neighbor Port ID

  • Lowest Local Port ID

• Cost values:

  • 10 Mbps: 100

  • 100 Mbps: 19

  • 1 Gbps: 4

  • 10 Gbps: 2

3. Designated Port Selection

• One Designated Port per segment (responsible for forwarding).

• All other ports: Non-Designated (Blocking)

🔹 STP Port States

1. Disabled – Admin down

2. Blocking – No data traffic, receives BPDUs

3. Listening – No traffic, sending/receiving BPDUs

4. Learning – Learns MACs, no data forwarding

5. Forwarding – Full forwarding

➡️ Transition: Listening (15s) → Learning (15s) → Forwarding

🔹 STP Timers

Timer	Default	Purpose
Hello	2s	BPDU interval
Forward Delay	15s	Time in Listening & Learning
Max Age	20s	Timeout for BPDU absence

➡️ Convergence Time: Up to 50 seconds

⚡ RSTP (Rapid STP) - IEEE 802.1w / Cisco Rapid PVST+

🔸 Key Improvements Over STP

Feature	STP	RSTP
Convergence Speed	Up to 50s	< 1s (best case)
Port States	5 (including disabled)	3 (Discarding, Learning, Forwarding)
Transition Method	Timer-based	Sync mechanism
Port Roles	Root, Designated, Non-designated	Root, Designated, Alternate, Backup
BPDU Behavior	Only Root sends	All switches send BPDUs
Topology Change Detection	Max Age (20s)	3 missed BPDUs (6s)

🔹 RSTP Port States

• Discarding = Blocking + Listening

• Learning

• Forwarding

🔹 RSTP Port Roles

Role	Purpose
Root	Best path to root bridge
Designated	One per segment, forwards traffic
Alternate	Backup path to root (on different switch)
Backup	Backup path on same segment (same switch)

🔹 RSTP Link Types

Type	Description	Behavior
Point-to-Point	Full-duplex links (default)	Fast transitions
Shared	Half-duplex/hub links	Falls back to STP behavior
Edge	End-device ports	Instant forwarding (PortFast)

• Configure with:

spanning-tree portfast
spanning-tree link-type point-to-point

🔹 RSTP Path Costs

RSTP uses both short and long path costs:

Speed	Short Cost	Long Cost
10 Mbps	100	2,000,000
100 Mbps	19	200,000
1 Gbps	4	20,000
10 Gbps	2	2,000
100 Gbps	—	200
1 Tbps	—	20
10 Tbps	—	2

Configure:

spanning-tree pathcost method {short | long}

🛠️ Cisco STP Toolkit Features (STP & RSTP Enhancements)

Feature	Function
PortFast	Enables fast transition on edge ports (end hosts only).
BPDU Guard	Shuts down a PortFast port if BPDU is received.
Root Guard	Prevents external switches from becoming root.
Loop Guard	Prevents non-designated ports from errantly forwarding.
BPDU Filter	Stops BPDUs on specific ports (risky if misused).

🔀 MSTP (Multiple Spanning Tree Protocol) - IEEE 802.1s

• Solves scalability issues in PVST+ by grouping VLANs into MST instances.

• Runs a single STP instance per group (not per VLAN).

• Uses RSTP mechanics for fast convergence.

• Example: VLANs 1–50 → MSTI1, VLANs 51–100 → MSTI2

🌐 ICCP STP Application (RFC 7727)

• Used in Provider Edge (PE) redundancy scenarios.

• Multiple PEs act as a virtual root bridge.

• Uses ICCP TLVs for config/state sync:

  • STP Connect, Root Time, Region Config, etc.

• Enables active-active operation, unlike VPLS (active-standby).

• If a PE fails → remaining PEs recalculate and reconverge.

📡 RSTP Management via MIB – RFC 4318

• Adds managed objects to Bridge MIB (RFC 4188).

• Important Objects:

  • dot1dStpVersion

  • dot1dStpPortAdminEdgePort

  • dot1dStpPortOperEdgePort

  • dot1dStpPortAdminPathCost

• Security Consideration: SNMPv3 recommended due to sensitivity of STP control.

🔁 STP vs RSTP — Summary Table

Feature	STP (802.1D / PVST+)	RSTP (802.1w / Rapid PVST+)
Convergence Time	Up to 50s	< 6s (often < 1s)
Port States	5	3
Port Roles	Root, Designated, Non-Designated	Root, Designated, Alternate, Backup
BPDU Handling	Only root bridge generates	All switches generate
Topology Change Detection	20s (Max Age)	6s (3 missed BPDUs)
Edge Port Behavior	Triggers TC	Does not trigger TC
Link Types	N/A	P2P, Shared, Edge
PortFast/Toolkit Support	Yes	Yes
VLAN Scaling	One STP per VLAN (PVST+)	Better with MSTP

✅ Interview Tips

• Be clear on Root Port vs Designated Port logic.

• Understand BPDU Guard vs Root Guard vs Loop Guard differences.

• Know the transition states and timers.

• Practice troubleshooting scenarios with:

show spanning-tree
debug spanning-tree events

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