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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 21 Configuring MSTP
Information About Configuring MSTP
the alternate before sending out a single prestandard BPDU, AY cannot detect that a prestandard switch
is connected to Y and continues to send standard BPDUs. 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 21-2 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 21-3 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, preventing the bridging loop.
Figure 21-3 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
