Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring RSTP (Rev. 03)
Rapid Spanning Tree Protocol (RSTP)
© 2008 Foundry Networks, Inc.
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Rapid Spanning Tree Protocol (RSTP)
The Rapid Spanning Tree Protocol is based on IEEE Std 802.1w and is part of
Amendment 2:
Rapid Reconfiguration
to IEEE Std 802.1D and IEEE Std 802.1t-2001.
RSTP performs the roles of the STP protocol considerably faster by utilizing point-to-point wiring
to provide rapid convergence of the spanning tree. With RSTP, the spanning tree may be
reconfigured in less than 1 second, as compared to 50 seconds required with the default STP
settings. This is critical in networks that carry voice, video, and other delay-sensitive traffic.
NOTE
The user can enable/disable RSTP per physical port only if it is enabled globally.
Overview
For information regarding the Spanning Tree timers, see “
Configuring Spanning Tree Protocol
(STP)
” chapter.
RSTP Port Roles
To provide rapid convergence of the spanning tree, RSTP assigns port roles and determines the
active topology. RSTP selects the device with the highest switch priority as is done in the STP.
RSTP assigns to each bridge port throughout the Bridged Local Area Network one of the roles
described in
Table 1.
Table 1: The Possible RSTP Port Role Assignments
Port Role
Description
Root port
Provides the best path (lowest cost) for packets forwarded from the switch
to the root switch.
Designated port
Connects to the designated switch, which provides the lowest path cost for
packets forwarded from that LAN to the root switch.
Alternate port
Offers an alternative path toward the root switch to that provided by the
current root port.
Backup port
Acts as a backup for the path provided by a designated port in the direction
of the leaves of the spanning tree. A backup port can exist only when two
ports are connected together in a loopback by a point-to-point link or when
a device has two or more connections to a shared LAN segment.
Disabled port
Blocked and has no role within the operation of the spanning tree.
The root and designated ports are included in the active topology, where as alternate and backup
ports are excluded.