Day 22 Rapid Spanning Tree Protocol (RSTP)

1. Rapid Spanning Tree Protocol (RSTP/802.1w) is an evolution of classic STP that significantly reduces network convergence time from up to 50 seconds down to just a few seconds by replacing timer-based transitions with an active negotiation mechanism between switches.

2. Rapid PVST+ is Cisco's proprietary enhancement to the 802.1w standard that runs a separate RSTP instance for each VLAN, enabling load balancing across different VLANs by blocking different ports per VLAN.

3. RSTP uses a bridge-to-bridge handshake mechanism that allows ports to move directly to the forwarding state without waiting through transitional states, which is fundamentally different from classic STP's timer-based approach.

4. RSTP incorporates several optional classic STP features by default, including UplinkFast, BackboneFast, and PortFast functionality, eliminating the need to manually configure these features.

RSTP Port States (3 states vs. 5 in classic STP)

RSTP Port Roles (4 roles)

RSTP Link Types

RSTP Port Costs (Updated from Classic STP)

Protocol Version Numbers

• Classic STP (802.1D): Protocol Version 0

• Rapid STP (802.1w): Protocol Version 2

RSTP Timers

• Neighbor loss detection: 3 missed BPDUs = 6 seconds (vs. 20 seconds in classic STP)

• Hello timer: 2 seconds (same as classic STP)

Optional Features Built into RSTP

1. Don't confuse link types with port roles or port states - They are separate concepts

   • Link types: Edge, Point-to-Point, Shared

   • Port roles: Root, Designated, Alternate, Backup

   • Port states: Discarding, Learning, Forwarding

2. Alternate vs Backup ports - Both are discarding, but:

   • Alternate = superior BPDU from different switch

   • Backup = superior BPDU from same switch (hub scenario only)

3. CLI shows "BLK" even in RSTP - The state is actually called "Discarding" in RSTP terminology

4. RSTP compatibility - RSTP is backward compatible with classic STP; interfaces connected to classic STP switches will operate in classic STP mode

5. Edge port configuration - Use spanning-tree portfast, NOT spanning-tree link-type edge

6. Protocol versions - Classic STP = 0, RSTP = 2 (not 1!)

1. RSTP Port Costs Pattern: Start with 2,000,000 for 10 Mbps, divide by 10 for each speed increase

   • 10 Mbps = 2,000,000

   • 100 Mbps = 200,000

   • 1 Gbps = 20,000

   • Pattern: 2-2-2-2-2-2 (just move decimal)

2. "A-B" for Non-Designated Roles:

   • Alternate = Another switch

   • Backup = Same Box (same switch)

3. RSTP States = "DLF": Discarding → Learning → Forwarding

4. 3-3-3 Rule for RSTP:

   • 3 port states

   • 3 missed BPDUs = neighbor lost

   • 3 link types

5. "Evolution, not Revolution": RSTP keeps same root bridge, root port, and designated port election rules

Determining Alternate vs Backup Port

• If SW3 receives a superior BPDU on G0/1 from SW2 → Alternate port (different switch)

• If SW2 receives its own BPDU back through a hub on G0/2 → Backup port (same switch)

Root Port Selection Through a Hub

• Hub doesn't participate in STP (no cost added)

• If costs are equal through hub, use neighbor bridge ID as tiebreaker

• Then use neighbor port ID if still tied

Classic STP uses timer-based transitions (30-50 seconds), while RSTP uses a bridge-to-bridge handshake/negotiation mechanism allowing ports to move directly to forwarding (few seconds).

1. Discarding (combines Disabled, Blocking, Listening)

2. Learning

3. Forwarding

1. Root Port

2. Designated Port

3. Alternate Port

4. Backup Port

A discarding port that receives a superior BPDU from another switch. It functions as a backup to the root port and can immediately become the root port if the current root port fails.

A discarding port that receives a superior BPDU from another interface on the same switch. This only occurs when connected to a hub. It serves as a backup for a designated port.

• 10 Mbps = 2,000,000

• 100 Mbps = 200,000

• 1 Gbps = 20,000

• 10 Gbps = 2,000

• Classic STP: Version 0

• RSTP: Version 2

3 missed BPDUs = 6 seconds (compared to 20 seconds/10 hello intervals in classic STP)

1. PortFast (edge ports)

2. UplinkFast (alternate port becomes root port immediately)

3. BackboneFast (rapid response to indirect failures)

1. Edge - connected to end hosts (full duplex)

2. Point-to-Point - direct switch-to-switch (full duplex)

3. Shared - connected to hub (half duplex)

Use the command spanning-tree portfast on the interface (same as classic STP PortFast)

Classic Spanning Tree Protocol - original STP with one instance for all VLANs, cannot load balance

Rapid Spanning Tree Protocol - faster convergence than 802.1D, but still only one instance for all VLANs

Multiple Spanning Tree Protocol (MSTP) - groups multiple VLANs into instances for load balancing

Cisco's upgrade to 802.1w that runs a separate RSTP instance per VLAN, allowing load balancing by blocking different ports in each VLAN

The RSTP switch interfaces connected to the classic STP switch will operate in classic STP mode (backward compatible), while other interfaces remain in RSTP mode.

ALL switches originate and send their own BPDUs from designated ports (unlike classic STP where only root bridge originates BPDUs)

• Classic STP: 2 bits (1st and 8th)

• RSTP: All 8 bits (used for negotiation process)

It flushes (deletes) all MAC addresses learned on that interface since it can no longer reach destinations through that interface.

Rapid PVST+ (rapid-pvst)

spanning-tree mode rapid-pvst

It shows "rstp" as the protocol

MSTP can group multiple VLANs into instances (e.g., VLANs 1-100 in instance 1), making it easier to manage than configuring separate root bridges for each VLAN in Rapid PVST+.

The interface with the lowest port ID becomes the designated port; the other becomes the backup port.

No, Cisco devices run the industry standard 802.1s MSTP.

RSTP's negotiation mechanism and built-in features (UplinkFast, BackboneFast) make the listening state unnecessary - ports can move directly to Learning or Forwarding when appropriate.

Duplex mode - full duplex = point-to-point, half duplex = shared. The switch auto-detects this.

spanning-tree link-type point-to-point

spanning-tree link-type shared

Rapid Spanning Tree (RSTP/Rapid PVST+)