Cisco Catalyst 2960 Series, Configuration Manual

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We recommend that you minimize the interaction between standard and prestandard MST implementations. Note
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.
This figure illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root
switch, 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.
Figure 14: Detecting Unidirectional Link Failure
MSTP 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 active
switchstack master.
The active switchstack master 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 switchstack master 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 switch that does not support MSTP is added to a switch stack that does support MSTP or the reverse, the
switch is put into a version mismatch state. If possible, the switch is automatically upgraded or downgraded
to the same version of software that is running on the switch stack.
When a new switch joins the stack, it sets its switch ID to the switch ID. If the newly added switch has the
lowest ID and if the root path cost is the same among all stack members, the newly added switch becomes
the stack root. A topology change occurs if the newly added switch contains a better root port for the switch
stack or a better designated port for the LAN connected to the stack. The newly added switch causes a topology
change in the network if another switch connected to the newly added switch changes its root port or designated
ports.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About MSTP