Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 436)

406 Multi-Service IronWare Switching Configuration Guide
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Bridges and bridge port roles
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IEEE 802.1W-2001 RSTP provides rapid traffic reconvergence for point-to-point links within a few
milliseconds (< 500 milliseconds), following the failure of a bridge or bridge port.
This reconvergence occurs more rapidly than the reconvergence provided by the IEEE 802.1D
Spanning Tree Protocol or by RSTP Draft 3 because:
• STP requires a newly selected Root port to go through listening and learning stages before
traffic convergence can be achieved. The STP traffic convergence time is calculated using the
following formula:
2 x FORWARD_DELAY + BRIDGE_MAX_AGE.
• Convergence in RSTP bridges 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.
NOTE
The rapid convergence will not occur on ports connected to shared media devices, such as hubs. To
take advantage of the rapid convergence provided by RSTP, make sure to explicitly configure all
point-to-point links in a topology.
Bridges and bridge port roles
A bridge in an RSTP 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 BPDU (RSTp packet):
• Root bridge ID
• Path cost value
• Transmitting bridge ID
• Designated port ID
RSTP algorithm uses this information to determine if the RST BPDU received by a port is superior to
the RST BPDU that the port transmits. The two values are compared in the order as given above,
starting with the Root bridge ID. The RST BPDU with a lower value is considered superior. The
superiority and inferiority of the RST BPDU is used to assign a role to a port.
If the value of the received RST BPDU is the same as that of the transmitted RST BPDU, then the
port ID in the RST BPDUs are compared. The RST BPDU with the lower port ID is superior. Port roles
are then calculated appropriately.
The port’s role is included in the BPDU that it transmits. The BPDU transmitted by an RSTP port is
referred to as an RST BPDU, while it is operating in RSTP mode.
Ports can have one of the following roles:
• Root – Provides the lowest cost path to the root bridge from a specific bridge
• Designated – Provides the lowest cost path to the root bridge from a LAN to which it is
connected
• Alternate – Provides an alternate path to the root bridge when the root port goes down
• Backup – Provides a backup to the LAN when the Designated port goes down
• Disabled – Has no role in the topology