Detecting Unidirectional Link Failure; Port Cost And Port Priority - Cisco Nexus 7000 Series Configuration Manual

Detecting Unidirectional Link Failure

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.
Detecting Unidirectional Link Failure
Currently, this feature is not present in the IEEE MST standard, but it is included in the standard-compliant
implementation; it is based on the dispute mechanism. 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. This
feature is based on the dispute mechanism.
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.
This 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.
Figure 15: Detecting a Unidirectional Link Failure

Port Cost and Port Priority

Spanning tree uses port costs to break a tie for the designated port. Lower values indicate lower port costs,
and spanning tree chooses the least costly path. Default port costs are taken from the bandwidth of the interface,
as follows:
• 10 Mbps—2,000,000
• 100 Mbps—200,000
• 1 Gigabit Ethernet—20,000
• 10 Gigabit Ethernet—2,000
You can configure the port costs in order to influence which port is chosen.
MST always uses the long path-cost calculation method, so the range of valid values is between 1 and
Note
200,000,000.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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Configuring MST Using Cisco NX-OS