Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 310)

Spanning tree reconvergence time
Spanning tree reconvergence using 802.1W Draft 3 can occur within one second.
After the spanning tree reconverges following the topology change, traffic also must reconverge on all
the bridges attached to the spanning tree. This is true regardless of whether 802.1W Draft 3 or
standard STP is used to reconverge the spanning tree.
Traffic reconvergence happens after the spanning tree reconvergence, and is achieved by flushing the
Layer 2 information on the bridges:
• Following 802.1W Draft 3 reconvergence of the spanning tree, traffic reconvergence occurs in the
time it takes for the bridge to detect the link changes plus the STP maximum age set on the bridge.
• If standard STP reconvergence occurs instead, traffic reconvergence takes two times the forward
delay plus the maximum age.
NOTE
802.1W Draft 3 does not apply when a failed root port comes back up. When this happens, standard
STP is used.
802.1W configuration considerations
802.1W Draft 3 is disabled by default. To ensure optimal performance of the feature before you enable
it,do the following:
• Configure the bridge priorities so that the root bridge is one that supports 802.1W Draft 3. (Use a
Brocade device or third-party device that supports 802.1W Draft 3.)
• Change the forwarding delay on the root bridge to a value lower than the default 15 seconds.
Brocade recommends a value from 3 - 10 seconds. The lower forwarding delay helps reduce
reconvergence delays in cases where 802.1W Draft 3 is not applicable, such as when a failed root
port comes back up.
• Configure the bridge priorities and root port costs so that each device has an active path to the root
bridge if its root port becomes unavailable. For example, port 3/4 is connected to port 2/4 on Switch
2, which has the second most favorable bridge priority in the spanning tree.
NOTE
If reconvergence involves changing the state of a root port on a bridge that supports 802.1D STP but
not 802.1W Draft 3, then reconvergence still requires the amount of time it takes for the ports on the
802.1D bridge to change state to forwarding (as needed), and receive BPDUs from the root bridge for
the new topology.
Enabling 802.1W Draft 3
802.1W Draft 3 is disabled by default. The procedure for enabling the feature differs depending on
whether single STP is enabled on the device.
NOTE
STP must be enabled before you can enable 802.1W Draft 3.
Spanning tree reconvergence time
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