Cisco WS-C2980G-A Software Guide - Page 111

Chapter 7 Configuring Spanning Tree Understanding How MST Works MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST feature has these characteristics: • MST runs a variant of spanning tree called Internal Spanning Tree (IST). IST augments the Common Spanning Tree (CST) information with internal information about the MST region. The MST region appears as a single bridge to adjacent Single Spanning Tree (SST) and MST regions. • A bridge running MST provides interoperability with single spanning tree bridges as follows: - MST bridges run a variant of STP (IST) that augments the Common Spanning Tree (CST) information with internal information about the MST region. - IST connects all the MST bridges in the region and appears as a subtree in the CST that encompasses the whole bridged domain. The MST region appears as a virtual bridge to adjacent SST bridges and MST regions. - The collection of ISTs in each MST region, the CST that interconnects the MST regions, and the SST bridges define Common and Internal Spanning Tree (CIST). CIST is the same as an IST inside an MST region and the same as CST outside an MST region. The STP, RSTP, and MSTP together elect a single bridge as the root of CIST. • MST establishes and maintains additional spanning trees within each MST region. These spanning trees are referred to as MST instances (MSTIs). The IST is numbered 0, and the MSTIs are numbered 1, 2, 3,... and so on. Any given MSTI is local to the MST region that is independent of MSTIs in another region, even if the MST regions are interconnected. MST instances combine with the IST at the boundary of MST regions to become the CST as follows: - Spanning tree information for an MSTI is contained in an MSTP record (M-record). M-records are always encapsulated within MST BPDUs (MST BPDUs). The original spanning trees computed by MSTP are called M-trees. M-trees are active only within the MST region. M-trees merge with the IST at the boundary of the MST region and form the CST. • MST provides interoperability with PVST+ by generating PVST+ BPDUs for the non-CST VLANs. • MST supports some of the PVST+ extensions in MSTP as follows: - UplinkFast and BackboneFast are not available in MST mode; they are part of RSTP. - PortFast is supported. - BPDU filtering and BPDU guard are supported in MST mode. - Loop guard and root guard are supported in MST. MST preserves the VLAN 1 disabled functionality except that BPDUs are still transmitted in VLAN 1. - MST switches behave as if MAC reduction is enabled. - For private VLANs, secondary VLANs are mapped to the same instance as the primary. Note the following guidelines when using MST: • Do not disable spanning tree on any VLAN in any of the PVST bridges. • Ensure that all PVST spanning tree root bridges have lower (numerically higher) priority than the CST root bridge. • Do not use PVST bridges as the root of CST. • Ensure that trunks carry all of the VLANs that are mapped to an instance or do not carry any VLANs at all. 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 7-15