Blade ICE RACKSWITCH G8124-E - Manual (Page 124)

BLADEOS 6.5.2 Application Guide
124 Chapter 8: Spanning Tree Protocols BMD00220, October 2010
Rapid Spanning Tree Protocol
Note – Rapid Spanning Tree Protocol (RSTP) is enabled by default on the G8124.
RSTP provides rapid convergence of the Spanning Tree and provides the fast re-configuration
critical for networks carrying delay-sensitive traffic such as voice and video. RSTP significantly
reduces the time to reconfigure the active topology of the network when changes occur to the
physical topology or its configuration parameters. RSTP reduces the bridged-LAN topology to a
single Spanning Tree.
RSTP was originally defined in IEEE 802.1w (2001) and was later incorporated into
IEEE 802.1D (2004), superseding the original STP standard.
RSTP parameters apply only to Spanning Tree Group (STG) 1. The STP/PVST+ mode STGs 2-128
are not used when the switch is placed in RSTP mode. Although many of the other STP/PVST+
options apply to RSTP as well, there are also new STP parameters to support RSTP, and some
values for existing parameters are different.
RSTP is compatible with devices that run IEEE 802.1D (1998) Spanning Tree Protocol. If the
switch detects IEEE 802.1D (1998) BPDUs, it responds with IEEE 802.1D (1998)-compatible data
units. RSTP is not compatible with Per-VLAN Rapid Spanning Tree (PVRST) protocol.
Port State Changes
The port state controls the forwarding and learning processes of Spanning Tree. In RSTP, the port
state has been consolidated to the following: discarding, learning, and forwarding.
Table 12
compares the port states between STP/PVST+ mode and RSTP mode.
Due to Spanning Tree’s sequence of discarding, learning, and forwarding, considerable delays may
occur while paths are being resolved. To mitigate delays, ports defined as edge ports (
“Port Type
and Link Type” on page 131 ) may bypass the Discarding and Learning states, and enter directly into
the Forwarding state.
Table 12 RSTP vs. STP Port states
Operational Status STP Port State RSTP Port State
Enabled Blocking Discarding
Enabled Listening Discarding
Enabled Learning Learning
Enabled Forwarding Forwarding
Disabled Disabled Discarding