204 G8264 Application Guide for ENOS 8.4Multiple Spanning Tree ProtocolMultiple Spanning Tree Protocol (MSTP) extends Rapid Spanning Tree Protocol(RSTP), allowing multiple Spanning Tree Groups (STGs) which may each includemultiple VLANs. MSTP was originally defined in IEEE 802.1s (2002) and was laterincluded in IEEE 802.1Q (2003).In MSTP mode, the G8264 supports up to 32 instances of Spanning Tree,corresponding to STGs 1‐32, with each STG acting as an independent,simultaneous instance of RSTP.MSTP allows frames assigned to different VLANs to follow separate paths, witheach path based on an independent Spanning Tree instance. This approachprovides multiple forwarding paths for data traffic, thereby enablingload‐balancing, and reducing the number of Spanning Tree instances required tosupport a large number of VLANs.Due to Spanning Tree’s sequence of discarding, learning, and forwarding, lengthydelays may occur while paths are being resolved. Ports defined as edge/portfastports (“Port Type and Link Type” on page 208) bypass the Discarding andLearning states, and enter directly into the Forwarding state.Note: In MSTP mode, Spanning Tree for the management ports is turned off bydefault.MSTP RegionA group of interconnected bridges that share the same attributes is called an MSTregion. Each bridge within the region must share the following attributes: Alphanumeric name Revision number VLAN‐to STG mapping schemeMSTP provides rapid re‐configuration, scalability and control due to the supportof regions, and multiple Spanning‐Tree instances support within each region.Common Internal Spanning TreeThe Common Internal Spanning Tree (CIST) or MST0 provides a common form ofSpanning Tree Protocol, with one Spanning‐Tree instance that can be usedthroughout the MSTP region. CIST allows the switch to interoperate with legacyequipment, including devices that run IEEE 802.1D (1998) STP.CIST allows the MSTP region to act as a virtual bridge to other bridges outside ofthe region, and provides a single Spanning‐Tree instance to interact with them.CIST port configuration includes Hello time, path‐cost, and interface priority.These parameters do not affect Spanning Tree Groups 1‐32. They apply only whenthe CIST is used.
