Detecting Unidirectional Link Failure - Cisco Catalyst 2975 Software Configuration Manual

Understanding MSTP
Figure 17-2
We recommend that you minimize the interaction between standard and prestandard MST
Note
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
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
Switch
Catalyst 2975 Switch Software Configuration Guide
17-8
Standard and Prestandard Switch Interoperation
Segment X
Switch A
Segment Y
illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the
Detecting Unidirectional Link Failure
Superior
BPDU
A
Inferior BPDU,
Designated + Learning bit set
MST
Region
Switch B
Switch
B
Chapter 17 Configuring MSTP
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