RSTP Rapid Spanning Tree Protocol

Last updated on Jun 20, 2023

Rapid Spanning Tree Protocol (RSTP) is an extension of the Spanning Tree Protocol (STP) used in computer networks to prevent loops and create loop-free paths in Ethernet networks. RSTP improves the convergence time of STP, making it faster and more efficient in adapting to changes in the network topology.

Here's a detailed explanation of RSTP and its key features:

Background:

Spanning Tree Protocol (STP) was developed to address the issue of loops in Ethernet networks. It creates a loop-free logical topology by selectively blocking redundant paths. However, the original STP has some limitations, including slow convergence time when there are network changes like link failures or additions.

Rapid Spanning Tree Protocol (RSTP):

RSTP, also known as IEEE 802.1w, was introduced as an enhancement to STP to improve convergence time. It retains the fundamental principles of STP but introduces several changes and enhancements to achieve faster convergence.

Key Features of RSTP:

a. Port States:

RSTP introduces three port states, compared to five port states in STP. The states are:

Discarding (alternate to STP's Blocking)
Learning
Forwarding

b. Roles:

Root Bridge: The switch with the lowest bridge ID becomes the root bridge. All other switches in the network elect root ports that provide the shortest path to the root bridge.
Designated Bridge: The switch with the lowest path cost to the root bridge is elected as the designated bridge for each network segment. It forwards traffic to and from the root bridge.
Alternate Port: A backup port that provides an alternate path to the root bridge. It transitions to the forwarding state if the root port fails.
Backup Port: A port that is ready to become an alternate port if the current designated port fails.

c. Port Roles:

Root Port: The port on a non-root bridge that provides the shortest path to the root bridge. It is in the forwarding state.
Designated Port: The port on a segment that is the designated bridge for that segment. It is in the forwarding state.
Discarding Port: A port that is not used for forwarding frames. It discards frames and does not participate in the data plane.

d. Link Types:

Point-to-Point Link: A link that connects only two switches. It allows the port to transition to the forwarding state immediately.
Shared Link: A link that connects multiple switches. It requires the port to go through the learning and forwarding states.

e. Rapid Convergence:

RSTP achieves faster convergence by reducing the time required to transition from the blocking state to the forwarding state.
Topology changes are detected faster through the use of periodic BPDU (Bridge Protocol Data Unit) messages.
RSTP uses the concept of edge ports that bypass the learning and listening states, immediately transitioning to the forwarding state.

f. Backward Compatibility:

RSTP is backward compatible with STP. If an RSTP-capable switch detects a non-RSTP switch, it falls back to the legacy STP behavior for that segment.
The Rapid Transition feature allows immediate transition to the forwarding state for known RSTP-capable devices connected to an RSTP-enabled switch.

RSTP and Multiple VLANs:

RSTP operates on a per-VLAN basis, which means each VLAN has its own spanning tree instance. This allows RSTP to provide faster convergence for individual VLANs while maintaining compatibility with STP for non-RSTP VLANs.

Overall, RSTP improves the convergence time of spanning tree protocols, making it more suitable for modern networks where rapid adaptability to network changes is crucial. Its enhancements and faster convergence make it an effective protocol for creating loop-free paths and ensuring the stability and efficiency of Ethernet networks.