Learning Objectives

• Understand how RSTP improves convergence time over classic STP
• Identify the three RSTP port roles and their purposes
• Explain MSTP's advantage for multi-VLAN environments

RSTP: Rapid Spanning Tree Protocol

Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, is not a different protocol — it's an evolution of STP that converges in 1-3 seconds instead of 30-50 seconds. RSTP is backwards-compatible with classic STP, so you can introduce it into existing networks.

RSTP achieves speed through three key changes. First, it eliminates the listening state — ports move directly from blocking to learning, but in practice transition to forwarding within a few BPDU intervals. Second, designated ports on point-to-point links can propose and agree to forward immediately through a handshake mechanism. Third, RSTP defines edge ports (ports connected to end devices) that skip the entire convergence process and go straight to forwarding.

RSTP Port Roles

RSTP uses five port roles, a superset of classic STP:

| RSTP Role | STP Equivalent | Description | |---|---|---| | Root Port (RP) | Root Port | Best path to the root bridge | | Designated Port (DP) | Designated Port | Best path from a segment toward the root | | Alternate Port | Blocking | Backup path to the root — instant failover | | Backup Port | Blocking | Redundant connection to the same segment | | Edge Port | Access Port | Connected to an end device — immediate forwarding |

The Alternate Port is the real innovation. In classic STP, a blocked port passively waits for BPDU timers to expire. In RSTP, an Alternate Port listens continuously and can begin forwarding immediately (within milliseconds) when it detects that the current Root Port has failed.

When a Root Port fails, the best Alternate Port becomes the new Root Port and immediately starts forwarding. There is no listening or learning delay because the port already knows the MAC table from prior passive learning.

RSTP Link Types

RSTP behaves differently depending on the link type, which it detects automatically:

• Point-to-point — full-duplex link between two switches. RSTP uses the proposal-agreement handshake for fast convergence. When a switch proposes itself as designated, the neighbor agrees immediately if it has no better path.
• Shared — half-duplex link (hub). RSTP falls back to classic STP behavior with timer-based transitions.
• Edge — port configured as edge (or detected via portfast equivalent). No BPDUs expected, immediate forwarding.

MSTP: Multiple Spanning Tree Protocol

MSTP (IEEE 802.1s) extends RSTP for networks with many VLANs. Without MSTP, each VLAN runs its own STP instance (PVST+ on Cisco). With 1000 VLANs, that's 1000 STP instances — significant CPU and BPDU overhead.

MSTP maps multiple VLANs to a single spanning tree instance. VLANs 1-100 might map to instance 1, VLANs 101-200 to instance 2, and so on. You design instances for optimal load balancing — different instances can have different root bridges, so traffic from different VLAN groups uses different paths.

The Cisco default is PVST+ (Per-VLAN Spanning Tree), where each VLAN runs its own STP instance. This allows per-VLAN load balancing but adds overhead. Rapid PVST+ is the same concept using RSTP. MSTP is the IEEE standard alternative.

Key Takeaways

• RSTP converges in 1-3 seconds vs 30-50 seconds for classic STP
• RSTP eliminates the listening state and uses proposal-agreement for fast transitions
• The Alternate Port provides instant failover when the Root Port fails
• Edge ports (access ports) skip convergence and forward immediately
• MSTP maps multiple VLANs to fewer spanning tree instances, reducing overhead
• PVST+ (Cisco) runs one STP per VLAN; MSTP (IEEE 802.1s) groups VLANs into instances