Detecting Unidirectional Link Failure; Port Cost And Port Priority - Cisco AJ732A - MDS 9134 Fabric Switch Configuration Manual

Information About MST
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
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. 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.
Figure 9-4
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 9-4
Switch

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:
•
•
•
•
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.
The system uses port priorities to break ties among ports with the same cost. A lower number indicates
a higher priority. The default port priority is 128. You can configure the priority to values between 0 and
224, in increments of 32.
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
9-8
shows a unidirectional link failure that typically creates a bridging loop. Switch A is the root
Detecting a Unidirectional Link Failure
Superior
BPDU
A
Inferior BPDU,
Designated + Learning bit set
10 Mbps—2,000,000
100 Mbps—200,000
1 Gigabit Ethernet—20,000
10 Gigabit Ethernet—2,000
Switch
B
Chapter 9 Configuring MST
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