Detecting Unidirectional Link Failure; Port Cost - Cisco AP775A - Nexus Converged Network Switch 5010 Configuration Manual

Information About Rapid PVST+

Detecting Unidirectional Link Failure

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 26: 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 17: Default Port Cost
Bandwidth
10 Mbps
100 Mbps
1 Gigabit Ethernet
10 Gigabit Ethernet
OL-16597-01
Short Path-cost Method of Port
Cost
100
19
4
2
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Detecting Unidirectional Link Failure
Long Path-cost Method of Port Cost
2,000,000
200,000
20,000
2,000
163