Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 309)

FIGURE 64 802.1W Draft 3 RSTP failover to alternate root port
In this example, port 3/3 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 3/4 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 3/4, 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
3/3 has the best cost to the root bridge and thus is selected by STP as the root port. Port 3/4 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.
Spanning Tree Protocol
FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide 309
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