user manual

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:
• 10 Mbps—2,000,000
• 100 Mbps—200,000
• 1 Gigabit Ethernet—20,000
• 10 Gigabit Ethernet—2,000
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
200,000,000.
Note
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 switch that runs MST supports a built-in protocol migration feature that enables it to interoperate with
802.1D STP switches. If this switch 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 switch 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 switch does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs
because it cannot detect whether the 802.1D switch has been removed from the link unless the 802.1D switch
is the designated switch. A switch might also continue to assign a boundary role to a port when the switch to
which this switch is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches), 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 switches can send either Version 0 configuration and topology change notification
(TCN) BPDUs or Version 3 MST BPDUs on a boundary port. A boundary port connects to a LAN, the
designated switch of which is either a single spanning tree switch or a switch with a different MST
configuration.
MST interoperates with the Cisco prestandard MSTP whenever it receives prestandard MSTP on an MST
port; no explicit configuration is necessary.
Note
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
96 OL-26590-01
Configuring Multiple Spanning Tree
Port Cost and Port Priority