HP Cisco Nexus 5000 Cisco Nexus 5000 Series Switch CLI Software Configuration - Page 168

Information About MST Chapter 9 Configuring MST Send feedback to [email protected] 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 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. Figure 9-4 Switch A Detecting a Unidirectional Link Failure Superior BPDU Switch B 184440 Inferior BPDU, Designated + Learning bit set 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. Note MST always uses the long path cost calculation method, so the range of valid values is between 1 and 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 OL-16597-01