Brocade TurboIron 24X Series Configuration Guide - Manual (Page 559)

Brocade TurboIron 24X Series Configuration Guide 525
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Configuring STP related features
802.1W Rapid Spanning Tree (RSTP)
Rapid Spanning Tree Protocol (RSTP), which was 802.1W Draft 3, provided only a subset of the
IEEE 802.1W standard; whereas the 802.1W RSTP feature provides the full standard. The
implementation of the 802.1W Draft 3 is referred to as RSTP Draft 3.
RSTP Draft3 will continue to be supported on devices for backward compatibility. However,
customers who are currently using RSTP Draft 3 should migrate to 802.1W.
The 802.1W feature provides rapid traffic reconvergence for point-to-point links within a few
milliseconds (0 – 500 milliseconds), following the failure of a bridge or bridge port. This
reconvergence occurs more rapidly than the reconvergence provided by the 802.1D Spanning Tree
Protocol (STP)) or by RSTP Draft 3.
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 802.1W, make sure to explicitly configure all
point-to-point links in a topology.
The convergence provided by the standard 802.1W protocol occurs more rapidly than the
convergence provided by previous spanning tree protocols because of the following:
• Classic or legacy 802.1D STP protocol requires a newly selected Root port to go through
listening and learning stages before traffic convergence can be achieved. The 802.1D traffic
convergence time is calculated using the following formula.
2 x FORWARD_DELAY + BRIDGE_MAX_AGE.
If default values are used in the parameter configuration, convergence can take up to 50
seconds. (In this document STP will be referred to as 802.1D.)
• RSTP Draft 3 works only on bridges that have Alternate ports, which are the precalculated
“next best root port”. (Alternate ports provide back up paths to the root bridge.) Although
convergence occurs from 0 – 500 milliseconds in RSTP Draft 3, the spanning tree topology
reverts to the 802.1D convergence if an Alternate port is not found.
• Convergence in 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.
Bridges and bridge port roles
A bridge in an 802.1W rapid spanning tree topology is assigned as the root bridge if it has the
highest priority (lowest bridge identifier) in the topology. Other bridges are referred to as non-root
bridges.
Unique roles are assigned to ports on the root and non-root bridges. Role assignments are based
on the following information contained in the Rapid Spanning Tree Bridge Packet Data Unit (RST
BPDU):
• Root bridge ID
• Path cost value
• Transmitting bridge ID
• Designated port ID