Quick Reference Guide

148 | Spanning Tree
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Port States
RSTP merges states from STP, leaving just three possible operational states. The 802.1D blocking and
disabled states are merged into the 802.1w discarding state. The 802.1D learning and listening states are
merged into the 802.1w learning state.
Port Costs
RSTP introduces new default port costs.
BPDU Format
RSTP has a unique BPDU format that uses all bits of the Flags field to communicate additional states. The
RSTP BPDUs act as a keep-alive between bridges, allowing for significantly faster link failure detection.
Convergence with RSTP
The faster convergence with RSTP results from the use of BPDUs as keep-alives between adjacent
switches, which establish the state before passing information to the downstream device. In contrast, the
pre-RSTP version of STP uses timers to allow BPDUs to flow from root to all leaves. Non-edge ports stay
a set time in listening and learning modes to gather all available BPDU information to decide the port state.
RSTP CLI Management
RSTP uses the same commands as basic STP (see Basic STP (802.1D) CLI Management on page 146).
Multiple Spanning-Tree Protocol (MSTP, IEEE 802.1s)
The default spanning tree mode in SFTOS is IEEE 802.1s (Multiple Spanning-Tree Protocol — MSTP),
which is backward-compatible with 802.1D (see Spanning Tree Protocol (STP, IEEE 802.1D) on
page 146) and 802.w (see Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w) on page 147).
SFTOS also supports each standalone mode — IEEE 802.1D, IEEE 802.1s, and IEEE 802.1w (see Setting
the STP Version Parameter on page 151).
MSTP allows LAN traffic to be channeled over different interfaces. MSTP also allows load balancing
without increasing CPU usage.
Rapid reconfiguration minimizes the time to recover from network outages, and increases network
availability.