Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 241

Chapter 17 Configuring STP and MST Understanding MST Topology Changes These are the differences between the RSTP and the 802.1D in handling spanning tree topology changes: • Detection-Unlike 802.1D in which any transition between the blocking and the forwarding state causes a topology change, only transitions from the blocking to the forwarding state cause a topology change with RSTP (only an increase in connectivity is considered a topology change). State changes on an edge port do not cause a topology change. When an RSTP switch detects a topology change, it deletes the learned information on all of its nonedge ports except on those from which it received the TC notification. • Notification-The RSTP does not use TCN BPDUs, unlike 802.1D. However, for 802.1D interoperability, an RSTP switch processes and generates TCN BPDUs. • Acknowledgement-When an RSTP switch receives a TCN message on a designated port from an 802.1D switch, it replies with an 802.1D configuration BPDU with the TCA bit set. However, if the TC-while timer (the same as the TC timer in 802.1D) is active on a root port connected to an 802.1D switch and a configuration BPDU with the TCA set is received, the TC-while timer is reset. This method of operation is only required to support 802.1D switches. The RSTP BPDUs never have the TCA bit set. • Propagation-When an RSTP switch receives a TC message from another switch through a designated or root port, it propagates the change to all of its nonedge, designated ports and to the root port (excluding the port on which it is received). The switch starts the TC-while timer for all such ports and flushes the information learned on them. • Protocol migration-For backward compatibility with 802.1D switches, RSTP selectively sends 802.1D configuration BPDUs and TCN BPDUs on a per-port basis. When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port and ignores the protocol type. If the switch receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. However, if the RSTP switch is using 802.1D BPDUs on a port and receives an RSTP BPDU after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port. Rapid-PVST Rapid-PVST uses the existing configuration for PVST+; however, Rapid-PVST uses RSTP to provide faster convergence. Independent VLANs run their own RSTP instance. Dynamic entries are flushed immediately on a per-port basis upon receiving a topology change. UplinkFast and BackboneFast configurations are ignored in Rapid-PVST mode; both features are included in RSTP. Understanding MST These sections describe MST: • MST Overview, page 17-18 • MST Regions, page 17-18 OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 17-17