Port Cost And Port Priority - Cisco Nexus 9000 Series Configuration Manual

Port Cost and Port Priority

Figure 13: Detecting a Unidirectional Link Failure
Port Cost and Port Priority
Spanning tree uses port costs to break a tie for the designated port. Lower values indicate lower port costs,
and spanning tree chooses the least costly path. Default port costs are taken from the bandwidth of the interface,
as follows:
• 1 Gigabit Ethernet—20,000
• 10 Gigabit Ethernet—2,000
• 40 Gigabit Ethernet—500
You can configure the port costs in order to influence which port is chosen.
MST always uses the long path-cost calculation method, so the range of valid values is between 1 and
Note
200,000,000.
The system uses port priorities to break ties among ports with the same cost. A lower number indicates a
higher priority. The default port priority is 128. You can configure the priority to values between 0 and 224,
in increments of 32.
Interoperability with IEEE 802.1D
A device that runs MST supports a built-in protocol migration feature that enables it to interoperate with
802.1D STP devices. If this device receives an 802.1D configuration BPDU (a BPDU with the protocol version
set to 0), it sends only 802.1D BPDUs on that port. In addition, an MST device can detect that a port is at the
boundary of a region when it receives an 802.1D BPDU, an MST BPDU (Version 3) associated with a different
region, or an 802.1w BPDU (Version 2).
However, the device does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs
because it cannot detect whether the 802.1D device has been removed from the link unless the 802.1D device
is the designated device. A device might also continue to assign a boundary role to a port when the device to
which this device is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring devices), enter the clear
spanning-tree detected-protocols command.
All Rapid PVST+ switches (and all 8021.D STP switches) on the link can process MST BPDUs as if they are
802.1w BPDUs. MST devices can send either Version 0 configuration and topology change notification (TCN)
Cisco Nexus 9000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
122
Configuring MST Using Cisco NX-OS