Cisco Systems N3KC3064TFAL3 Manual De Usuario
with a port that belongs to a different region, creating the possibility of receiving both internal and external
messages on a port (see the following figure).
messages on a port (see the following figure).
Figure 16: MST Boundary Ports
At the boundary, the roles of MST ports do not matter; the system forces their state to be the same as the IST
port state. If the boundary flag is set for the port, the MST port-role selection process assigns a port role to
the boundary and assigns the same state as the state of the IST port. The IST port at the boundary can take up
any port role except a backup port role.
port state. If the boundary flag is set for the port, the MST port-role selection process assigns a port role to
the boundary and assigns the same state as the state of the IST port. The IST port at the boundary can take up
any port role except a backup port role.
Spanning-Tree Dispute Mechanism
Currently, this feature is not present in the IEEE MST standard, but it is included in the standard-compliant
implementation. 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.
implementation. 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 a discarding state because disrupting
connectivity in case of inconsistency is preferable to opening a bridging loop.
connectivity in case of inconsistency is preferable to opening a bridging loop.
The following figure shows a unidirectional link failure that typically creates a bridging loop. Switch A is the
root bridge, and its BPDUs are lost on the link leading to switch B. Rapid PVST+ (802.1w) 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 that it sends and that switch B is the designated, not root port. As a result,
switch A blocks (or keeps blocking) its port, which prevents the bridging loop. The block is shown as an STP
dispute.
root bridge, and its BPDUs are lost on the link leading to switch B. Rapid PVST+ (802.1w) 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 that it sends and that switch B is the designated, not root port. As a result,
switch A blocks (or keeps blocking) its port, which prevents the bridging loop. The block is shown as an STP
dispute.
Figure 17: Detecting a Unidirectional Link Failure
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
OL-26590-01
95
Configuring Multiple Spanning Tree
Spanning-Tree Dispute Mechanism