802.1W Draft 3

As an alternative to full 802.1W, you can configure 802.1W Draft 3. 802.1W Draft 3 provides a subset of the RSTP capabilities described in the 802.1W STP specification.

802.1W Draft 3 support is disabled by default. When the feature is enabled, if a root port on a RUCKUS device that is not the root bridge becomes unavailable, the device can automatically Switch over to an alternate root port, without reconvergence delays. 802.1W Draft 3 does not apply to the root bridge, since all the root bridge ports are always in the forwarding state.

The following figure shows an example of an optimal STP topology. In this topology, all the non-root bridges have at least two paths to the root bridge (Switch 1 in this example). One of the paths is through the root port. The other path is a backup and is through the alternate port. While the root port is in the forwarding state, the alternate port is in the blocking state.

Figure 1. 802.1W Draft 3 RSTP ready for failover

If the root port on a Switch becomes unavailable, 802.1W Draft 3 immediately fails over to the alternate port, as shown in the following figure.

Figure 2. 802.1W Draft 3 RSTP failover to alternate root port

In this example, port 1/2/1 on Switch 3 has become unavailable. In standard STP (802.1D), if the root port becomes unavailable, the Switch must go through the listening and learning stages on the alternate port to reconverge with the spanning tree. Thus, port 1/2/2 must go through the listening and learning states before entering the forwarding state and thus reconverging with the spanning tree.

802.1W Draft 3 avoids the reconvergence delay by calculating an alternate root port, and immediately failing over to the alternate port if the root port becomes unavailable. The alternate port is in the blocking state as long as the root port is in the forwarding state, but moves immediately to the active state if the root port becomes unavailable. Thus, using 802.1W Draft 3, Switch 3 immediately fails over to port 1/2/2, without the delays caused by the listening and learning states.

802.1W Draft 3 selects the port with the next-best cost to the root bridge. For example, on Switch 3, port 1/2/1 has the best cost to the root bridge and thus is selected by STP as the root port. Port 1/2/2 has the next-best cost to the root bridge, and thus is selected by 802.1W Draft 3 as the alternate path to the root bridge.

Once a failover occurs, the Switch no longer has an alternate root port. If the port that was an alternate port but became the root port fails, standard STP is used to reconverge with the network. You can minimize the reconvergence delay in this case by setting the forwarding delay on the root bridge to a lower value. For example, if the forwarding delay is set to 15 seconds (the default), change the forwarding delay to a value from 3 - 10 seconds.

During failover, 802.1W Draft 3 flushes the MAC addresses leaned on the unavailable root port, selects the alternate port as the new root port, and places that port in the forwarding state. If traffic is flowing in both directions on the new root port, addresses are flushed (moved) in the rest of the spanning tree automatically.