Cisco Catalyst 2960 Series, Configuration Manual

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For example, in the figure below, Switch A is the root bridge and Switch B is the designated port. BPDUs
from Switch A are lost on the link leading to switch B.
Figure 17: Detecting Unidirectional Link Failure
Since Rapid PVST+ (802.1w) and MST BPDUs include the role and state of the sending port, Switch A detects
(from the inferior BPDU), that switch B does not react to the superior BPDUs it sends, because switch B has
the role of a designated port and not the root bridge. As a result, switch A blocks (or keeps blocking) its port,
thus preventing the bridging loop.
Note these guidelines and limitations relating to the dispute mechanism:
• It works only on switches running RSTP or MST (the dispute mechanism requires reading the role and
state of the port initiating BPDUs).
• It may result in loss of connectivity. For example, in the figure below, Bridge A cannot transmit on the
port it elected as a root port. As a result of this situation, there is loss of connectivity (r1 and r2 are
designated, a1 is root and a2 is alternate. There is only a one way connectivity between A and R).
Figure 18: Loss of Connectivity
• It may cause permanent bridging loops on shared segments. For example, in the figure below, suppose
that bridge R has the best priority, and that port b1 cannot receive any traffic from the shared segment
1 and sends inferior designated information on segment 1. Both r1 and a1 can detect this inconsistency.
However, with the current dispute mechanism, only r1 will revert to discarding while the root port a1
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About MSTP