Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 8 Configuring Rapid PVST+
Information About Rapid PVST+
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
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 (see Table 8-4). If a loop occurs, STP considers the port cost when selecting
a LAN interface to put into the forwarding state.
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.
Inferior BPDU,
Designated + Learning bit set
Superior
BPDU
Switch
A
Switch
B
184440
Table 8-4 Default Port Cost
Bandwidth
Short Path-cost Method
of Port Cost
Long Path-cost Method
of Port Cost
10 Mbps 100 2,000,000
100 Mbps 19 200,000
1 Gigabit Ethernet 4 20,000
10 Gigabit Ethernet 2 2,000