Cisco Systems N3KC3048TP1GE, N3KC3064TFAL3 manual Spanning-Tree Dispute Mechanism, Port Cost

Configuring Rapid PVST+

Understanding Rapid PVST+

Processing Inferior BPDU Information

An inferior BPDU is a BPDU with root information (such as a higher switch ID or higher path cost) that is inferior to what is currently stored for the port.

If a designated port receives an inferior BPDU, it immediately replies with its own information.

Spanning-Tree 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.

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.

The following figure 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 13: Detecting Unidirectional Link Failure

Port Cost

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. If a loop occurs, STP considers the port cost when selecting a LAN interface to put into the forwarding state.

Table 9: Default Port Cost
Bandwidth	Short Path-cost Method of Port	Long Path-cost Method of Port Cost
	Cost
10 Mbps	100	2,000,000

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
76	OL-26590-01