© Copyright IBM Corp. 2011 115
Chapter 10. Spanning Tree Protocols
When multiple paths exist between two points on a network, Spanning Tree Protocol
(STP), or one of its enhanced variants, can prevent broadcast loops and ensure that
the RackSwitch G8000 (G8000) uses only the most efficient network path.
This chapter covers the following topics:
•
“Spanning Tree Protocol Modes” on page 115
•
“Global STP Control” on page 116
•
“PVRST Mode” on page 116
•
“Rapid Spanning Tree Protocol” on page 126
•
“Multiple Spanning Tree Protocol” on page 127
•
“Port Type and Link Type” on page 129
Spanning Tree Protocol Modes
IBM Networking OS 6.8 supports the following STP modes:
•
Rapid Spanning Tree Protocol (RSTP)
IEEE 802.1D (2004) RSTP allows devices to detect and eliminate logical loops
in a bridged or switched network. When multiple paths exist, STP configures the
network so that only the most efficient path is used. If that path fails, STP
automatically configures the best alternative active path on the network to
sustain network operations. RSTP is an enhanced version of IEEE 802.1D
(1998) STP, providing more rapid convergence of the Spanning Tree network
path states on STG 1.
See “Rapid Spanning Tree Protocol” on page 126 for details.
•
Per-VLAN Rapid Spanning Tree (PVRST)
PVRST mode is based on RSTP to provide rapid Spanning Tree convergence,
but supports instances of Spanning Tree, allowing one STG per VLAN. PVRST
mode is compatible with Cisco R-PVST/R-PVST+ mode.
PVRST is the default Spanning Tree mode on the G8000. See “PVRST Mode”
on page 116 for details.
•
Multiple Spanning Tree Protocol (MSTP)
IEEE 802.1Q (2003) MSTP provides both rapid convergence and load balancing
in a VLAN environment. MSTP allows multiple STGs, with multiple VLANs in
each.
MSTP is supported in stand-alone (non-stacking) mode only.
See “Multiple Spanning Tree Protocol” on page 127 for details.
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