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17-8
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 17 Configuring MSTP
Understanding MSTP
Figure 17-2 Standard and Prestandard Switch Interoperation
Note We recommend that you minimize the interaction between standard and prestandard MST
implementations.
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 discarding state because
disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.
Figure 17-3 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, thus preventing the bridging loop.
Figure 17-3 Detecting Unidirectional Link Failure
Segment X
MST
Region
Segment Y
92721
Switch A
Switch B
Inferior BPDU,
Designated + Learning bit set
Superior
BPDU
Switch
A
Switch
B
92722