• Propagation—When an RSTP device receives a TC message from another device 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 device starts the TC-while timer for all such ports and flushes the information learned on them. • Protocol migration—For backward compatibility with IEEE 802.1D devices, RSTP selectively sends IEEE 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 device processes all BPDUs received on that port and ignores the protocol type. If the device receives an IEEE 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an IEEE 802.1D device and starts using only IEEE 802.1D BPDUs. However, if the RSTP device is using IEEE 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. Protocol Migration Process A device running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy IEEE 802.1D devices.
If this device receives a legacy IEEE 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP device also can detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (Version 3) associated with a different region, or an RST BPDU (Version 2). However, the device does not automatically revert to the MSTP mode if it no longer receives IEEE 802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy switch is the designated device.
A device also might continue to assign a boundary role to a port when the device to which it is connected has joined the region. Default Muliple Spanning Tree Protocol Configuration Table 3: Default MSTP Configuration Default Setting Feature Spanning-tree mode 32768 Device priority (configurable on a per-CIST port basis) 128 Spanning-tree port priority (configurable on a per-CIST port basis) Spanning-tree port cost (configurable on a per-CIST port basis) Hello time Forward-delay time 20 seconds Maximum-aging time 20 hops Maximum hop count Configuring Multiple Spanning-Tree Protocol Protocol Migration Process Configuring Multiple Spanning-Tree Protocol 15