RSTP — Rapid Spanning Tree

Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, is an enhancement of the original Spanning Tree Protocol (802.1D). Its primary goal is to provide faster convergence after a topology change—typically in sub-second time—compared to the 30-50 seconds required by classic STP. RSTP achieves this by redefining port roles, port states, and the BPDU exchange mechanism. It is fully backward compatible with 802.1D STP, meaning RSTP switches can interoperate with legacy STP switches, though they will fall back to slower convergence on those links.

RSTP uses the same BPDU format as STP but with a few key differences: - Version: RSTP BPDUs use version ID 2 (STP uses version 0). - Flags: RSTP uses all 8 flag bits in the BPDU, including Proposal, Agreement, Learning, Forwarding, Topology Change Acknowledgment, and Topology Change. In STP, only the Topology Change (TC) and TCA bits were used. - Type: RSTP BPDUs are sent with type 0x02 (configuration BPDU) but with the protocol version identifier set to 2.

RSTP switches send BPDUs every hello time (2 seconds) out of all ports, even if the port is blocking. This is a major departure from classic STP, where only designated ports send BPDUs. This constant hello allows neighbors to detect link failures within three missed hellos (6 seconds) rather than waiting for the Max Age timer (20 seconds).

RSTP defines the following port roles: - Root Port (RP): The port that has the best path to the root bridge. Same as STP. - Designated Port (DP): The port that has the best path from the segment to the root bridge. Same as STP. - Alternate Port (AP): A port that provides an alternate path to the root bridge—essentially a backup to the root port. In STP, this was a blocking port that was not a designated port. - Backup Port (BP): A port that provides a backup for a designated port—only possible when two ports connect to the same collision domain (e.g., hub). Rare in modern switched networks. - Disabled Port: A port that is administratively down.

RSTP reduces the number of port states from five (Blocking, Listening, Learning, Forwarding, Disabled) to three: - Discarding: The port does not forward frames and does not learn MAC addresses. This combines the old Blocking, Listening, and Disabled states. - Learning: The port does not forward frames but learns MAC addresses. - Forwarding: The port forwards frames and learns MAC addresses.

The core innovation in RSTP is the proposal/agreement (P/A) handshake, which allows a port to transition directly from Discarding to Forwarding without waiting for timers. Here's how it works: 1. A switch that wants to become the designated port on a link sends a Proposal BPDU (with the Proposal flag set) to the neighbor. 2. The neighbor, upon receiving the Proposal, immediately places its port into Discarding state (if it is currently in a learning/forwarding state) and sends back an Agreement BPDU (with the Agreement flag set). The neighbor also ensures that all its other ports are in sync (i.e., not creating a loop) before sending the Agreement. 3. The originating switch receives the Agreement and transitions its port to Forwarding immediately.

This handshake occurs within one round-trip time (milliseconds), eliminating the need for the Listening and Learning timer delays (15 seconds each in STP). The handshake propagates from the root bridge outward, ensuring a loop-free topology is built quickly.

RSTP introduces the concept of edge ports (equivalent to PortFast in Cisco terminology). An edge port is a port that connects to an end device (like a PC or server) and is assumed to never create a loop. Edge ports transition directly to Forwarding without any handshake or delay. If an RSTP BPDU is received on an edge port, it immediately loses its edge status and becomes a normal spanning-tree port.

RSTP also distinguishes between point-to-point links (full-duplex) and shared links (half-duplex). The P/A handshake only works on point-to-point links. On shared links, RSTP falls back to classic STP behavior (timer-based transitions).

When an RSTP switch detects a topology change (e.g., a link going down), it: 1. Starts a topology change (TC) while timer (default 4 seconds, twice the hello time). 2. Sets the TC flag in its BPDUs for the duration of the TC while timer. 3. Flushes MAC addresses learned on all ports except the port that received the TC BPDU.

This is much faster than STP, which uses a 35-second (Max Age + Forward Delay) mechanism to age out MAC addresses.

To verify RSTP operation on a Cisco switch, use the following commands:

Switch# show spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     0050.7966.6800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0050.7966.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 4         128.1    P2p
Gi0/1            Desg FWD 4         128.2    P2p
Gi0/2            Desg FWD 4         128.3    P2p
Gi0/3            Desg FWD 4         128.4    P2p

Notice the line Spanning tree enabled protocol rstp. The role column shows Desg (Designated), Root, Altn (Alternate), or Back (Backup). The state column shows FWD (Forwarding), LRN (Learning), or BLK (Discarding). The type column indicates P2p (point-to-point), Shr (shared), or Edge.

To see detailed RSTP information for a specific interface:

Switch# show spanning-tree interface gigabitethernet 0/0 detail

Port 1 (GigabitEthernet0/0) of VLAN0001 is designated forwarding
  Port path cost 4, Port priority 128, Port identifier 128.1.
  Designated root has priority 32769, address 0050.7966.6800
  Designated bridge has priority 32769, address 0050.7966.6800
  Designated port id is 128.1, designated path cost 0
  Timers: message age 0, forward delay 0, hold 0
  Number of transitions to forwarding state: 1
  Link type is point-to-point (auto)
  BPDU: sent 17, received 0
  RSTP: designated port, Proposal Agree flag set

RSTP interacts with: - PVST+: Cisco's Per-VLAN Spanning Tree Plus. With RSTP, Cisco uses Rapid PVST+ (PVST+ with RSTP), which runs a separate RSTP instance per VLAN. This is the default mode on modern Cisco switches. - MST: Multiple Spanning Tree (802.1s) uses RSTP as its underlying mechanism for rapid convergence within instances. - PortFast: Cisco's proprietary feature for edge ports; RSTP edge ports are equivalent. - BPDU Guard: Shuts down a port if a BPDU is received (used on edge ports). - BPDU Filter: Prevents sending/receiving BPDUs on a port (used on edge ports).

In a modern enterprise campus network, RSTP (specifically Rapid PVST+) is the default spanning-tree mode on Cisco Catalyst switches. For example, a three-tier network with access, distribution, and core layers often uses RSTP to ensure fast failover. When a link between a distribution switch and an access switch fails, RSTP can converge within a few hundred milliseconds, minimizing disruption to end users. This is critical for voice and video traffic that cannot tolerate 30+ second outages.

A common deployment scenario: An engineer configures all access ports as edge ports (PortFast) with BPDU Guard enabled. This prevents loops from misconfigured end devices and allows immediate forwarding. On uplinks between switches, the engineer relies on the P/A handshake for rapid transition. In a well-designed network, the root bridge is manually set to a core switch for stability. The engineer might also tune the hello time to 1 second for faster failure detection, though this increases CPU load.

Performance considerations: RSTP scales well with the number of VLANs because each VLAN runs its own instance (Rapid PVST+). However, with hundreds of VLANs, the CPU overhead of sending BPDUs for each VLAN can be significant. In such cases, MST (Multiple Spanning Tree) is preferred, which uses RSTP internally but maps multiple VLANs to fewer instances.

Misconfiguration consequences: If an engineer accidentally configures an uplink as an edge port, the switch will not expect BPDUs on that port. If a BPDU is received (e.g., from another switch), BPDU Guard will errdisable the port, causing an outage. Conversely, failing to configure an access port as an edge port causes a 30-second delay before the port forwards, annoying users who expect instant connectivity. Another common issue: if a link is half-duplex (e.g., due to auto-negotiation failure), RSTP falls back to timer-based convergence, causing slow failover.