The port BY is fixed in a boundary, and no load-balancing is possible between A and B. The same problem
exists on segment X, but B might transmit topology changes.
Figure 6: Standard and Prestandard Switch Interoperation
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 7: Detecting Unidirectional Link Failure
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
OL-29424-01 51
Configuring Multiple Spanning-Tree Protocol
IEEE 802.1s Implementation
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