Dell PowerConnect B-FCXs Configuration Manual page 309

Powerconnect b-series fcx

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The arrow shows the path

to the root bridge

Bridge priority = 2

Bridge priority = 6

Root port = 3/4

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

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

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.

PowerConnect B-Series FCX Configuration Guide

53-1002266-01

802.1W Draft 3 RSTP failover to alternate root port

Configuring STP related features

Bridge priority = 4

Root port = 2/2

Alternate = 2/3, 2/4

Bridge priority = 8

Root port = 4/4

Alternate = 4/3

Troubleshooting