RSTP Overview

Beginning with FastIron release 10.0.20a, Rapid Spanning Tree Protocol (RSTP) (IEEE IEEE 802.1w) replaces the Spanning Tree Protocol (STP) (IEEE 802.1D) standard. RSTP enables faster convergence times than STP. The RSTP is supported on all RUCKUS ICX devices.

Like STP, RSTP eliminates Layer 2 loops in networks by selectively blocking some ports and allowing other ports to forward traffic.

The RSTP is backward-compatible with STP. However, the faster convergence times are lost when interacting with legacy bridges.

Note: This rapid convergence will not occur on ports connected to shared media devices, such as hubs. To take advantage of the rapid convergence provided by IEEE 802.1w, make sure to explicitly configure all point-to-point links in a topology.

The convergence provided by the standard IEEE 802.1w protocol occurs more rapidly than the convergence provided by previous spanning tree protocols because of the following:

  • Classic or legacy IEEE 802.1D STP protocol requires a newly selected Root port to go through listening and learning stages before traffic convergence can be achieved. The IEEE 802.1D traffic convergence time is calculated using the following formula.

    2 x FORWARD_DELAY + BRIDGE_MAX_AGE

  • Convergence in IEEE 802.1w bridge is not based on any timer values. Rather, it is based on the explicit handshakes between Designated ports and their connected Root ports to achieve convergence in less than 500 milliseconds.

Beginning with FastIron release 10.0.20a, RSTP is enabled by default at the VLAN level.

You can enable or disable RSTP IEEE IEEE 802.1w on the following levels:

  • Globally: Affects all ports and port-based VLANs on the device.
  • Port-based VLAN: Affects all ports within the specified port-based VLAN. When you enable or disable RSTP within a port-based VLAN, the setting overrides the global setting. Thus, you can enable RSTP for the ports within a port-based VLAN even when RSTP is globally disabled, or disable the ports within a port-based VLAN when RSTP is globally enabled.
  • Individual port: Affects only the individual port. However, if you change the RSTP state of the link aggregation group (LAG) virtual interface, the change affects all ports in the LAG.

RSTP Configuration Modes

The following configuration modes apply while configuring RSTP:

  • Spanning-tree single IEEE 802.1w: This configuration can be enabled on systems running IEEE 802.1w. The single rapid spanning tree controls all 4000 VLANs. The VLAN can opt in and out of this single rapid spanning tree using the spanning-tree 802-1w command under the VLAN prompt. If there is a “spanning-tree 802-1w” configuration under the VLAN, that VLAN will be within that single IEEE 802.1w instance’s control, which implies that the VLAN traffic is subject to blocking or forwarding by that spanning tree instance.
  • Per-VLAN spanning tree: This configuration mode enables IEEE 802.1w (Rapid Spanning Tree Protocol) at the VLAN level individually.

RSTP Protection Enhancement

RSTP Protection provides the ability to prohibit an end station from initiating or participating in an RSTP topology change.

The RSTP detects and eliminates logical loops in a redundant network by selectively blocking some data paths (ports) and allowing only the best data paths to forward traffic.

In an RSTP environment, switches, end stations, and other Layer 2 devices use Bridge Protocol Data Units (BPDUs) to exchange information that RSTP will use to determine the best path for data flow. When a Layer 2 device is powered on and connected to the network, or when a Layer 2 device goes down, it sends out an RST BPDU, triggering an RSTP topology change.

In some instances, it is unnecessary for a connected device, such as an end station, to initiate or participate in an RSTP topology change. In this case, you can enable RSTP Protection on the RUCKUS port to which the end station is connected. RSTP Protection disables the ability of the connected device to initiate or participate in an RSTP topology change by dropping all BPDUs received from the connected device.

Parent topic: Rapid Spanning Tree Protocol