HP Cisco Nexus 5000 Cisco Nexus 5000 Series Switch CLI Software Configuration - Page 149

Chapter 8 Configuring Rapid PVST+ Information About Rapid PVST+ Send feedback to [email protected] 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 8-7 illustrates 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. The 802.1w-standard 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 it sends and that switch B is the designated, not root port. As a result, switch A blocks (or keeps blocking) its port, thus preventing the bridging loop. The block is shown as an STP dispute. Figure 8-7 Detecting Unidirectional Link Failure Switch A Superior BPDU Switch B 184440 Port Cost Inferior BPDU, Designated + Learning bit set Note Rapid PVST+ uses the short (16-bit) pathcost method to calculate the cost by default. With the short pathcost method, you can assign any value in the range of 1 to 65535. However, you can configure the switch to use the long (32-bit) pathcost method, which allows you to assign any value in the range of 1 to 200,000,000. You configure the pathcost calculation method globally. The STP port path-cost default value is determined from the media speed and path-cost calculation method of a LAN interface (see Table 8-4). If a loop occurs, STP considers the port cost when selecting a LAN interface to put into the forwarding state. Table 8-4 Default Port Cost Bandwidth 10 Mbps 100 Mbps 1 Gigabit Ethernet 10 Gigabit Ethernet Short Path-cost Method Long Path-cost Method of Port Cost of Port Cost 100 2,000,000 19 200,000 4 20,000 2 2,000 You can assign lower cost values to LAN interfaces that you want STP to select first and higher cost values to LAN interfaces that you want STP to select last. If all LAN interfaces have the same cost value, STP puts the LAN interface with the lowest LAN interface number in the forwarding state and blocks other LAN interfaces. On access ports, you assign port cost by the port. On trunk ports, you assign the port cost by the VLAN; you can configure the same port cost to all the VLANs on a trunk port. OL-16597-01 Cisco Nexus 5000 Series Switch CLI Software Configuration Guide 8-15