Detecting Unidirectional Link Failure:rapid Pvst; Port Cost - Cisco Nexus 7000 Series Configuration Manual

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

Detecting Unidirectional Link Failure:Rapid PVST+

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 using the Unidirectional Link Detection (UDLD) feature. This
feature is based on the dispute mechanism.
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.
This 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 that it sends and that switch B is the designated, not the root port. As a result, switch A blocks
(or keeps blocking) its port, which prevents the bridging loop.
Figure 10: Detecting Unidirectional Link Failure

Port Cost

Rapid PVST+ uses the short (16-bit) path-cost method to calculate the cost by default. With the short
Note
path-cost method, you can assign any value in the range of 1 to 65535. However, you can configure the
device to use the long (32-bit) path-cost method, which allows you to assign any value in the range of 1
to 200,000,000. You configure the path-cost calculation method globally.
This table shows how the STP port path-cost default value is determined from the media speed and path-cost
calculation method of a LAN interface.
Table 12: Default Port Cost
Bandwidth
10 Mbps
100 Mbps
1 Gigabit Ethernet
10 Gigabit Ethernet
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
104
Short Path-Cost Method of Port
Cost
100
19
4
2
Configuring Rapid PVST+ Using Cisco NX-OS
Long Path-Cost Method of Port Cost
2,000,000
200,000
20,000
2,000