Port Cost - Cisco N5010P-N2K-BE Software Configuration Manual

Chapter 8 Configuring Rapid PVST+
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
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
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
Switch

Port Cost

Rapid PVST+ uses the short (16-bit) pathcost method to calculate the cost by default. With the short
Note
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
a LAN interface to put into the forwarding state.
Table 8-4
Bandwidth
10 Mbps
100 Mbps
1 Gigabit Ethernet
10 Gigabit Ethernet
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
illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the
Detecting Unidirectional Link Failure
Superior
BPDU
A
Inferior BPDU,
Designated + Learning bit set
Default Port Cost
Short Path-cost Method
of Port Cost
100
19
4
2
Switch
B
Table 8-4). If a loop occurs, STP considers the port cost when selecting
Long Path-cost Method
of Port Cost
2,000,000
200,000
20,000
2,000
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Information About Rapid PVST+
8-15