Detecting Unidirectional Link Failure This feature is not yet present in the IEEE MST standard, but it is included in this Cisco IOS release. The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops. When a designated port detects a conflict, it keeps its role, but reverts to the discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop. Figure 3: Detecting Unidirectional Link Failure This figure illustrates a unidirectional link failure that typically creates a bridging loop.
Switch A is the root device, and its BPDUs are lost on the link leading to Switch B. RSTP and MST BPDUs include the role and state of the sending port. With this information, Switch A can detect that Switch B does not react to the superior BPDUs it sends and that Switch B is the designated, not root switch. As a result, Switch A blocks (or keeps blocking) its port, which prevents the bridging loop. Multiple Spanning Tree Protocol and Switch Stacks A switch stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same bridge ID for a given spanning tree. The bridge ID is derived from the MAC address of the device.
The active switch is the stack root when the stack is the root of the network and no root selection has been made within the stack. If the switch stack is the spanning-tree root and the active switch fails or leaves the stack, the standby switch becomes the new active switch, bridge IDs remain the same, and a spanning-tree reconvergence might occur. If a device that does not support MSTP is added to a switch stack that does support MSTP or the reverse, the device is put into a version mismatch state. If possible, the device is automatically upgraded or downgraded to the same version of software that is running on the switch stack.
Interoperability with IEEE 802.1D Spanning Tree Protocol 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 MSTP BPDU (Version 3) associated with a different region, or an RSTP 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 might also continue to assign a boundary role to a port when the device to which this device is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring devices), use the clear spanning-tree detected-protocols privileged EXEC command. Configuring Multiple Spanning-Tree Protocol Detecting Unidirectional Link Failure Configuring Multiple Spanning-Tree Protocol 9