Cisco N5010P-N2K-BE, Software Configuration Manual

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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
9-9
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 9 Configuring MST
Configuring MST
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.
Note
MST interoperates with the Cisco prestandard MSTP whenever it receives prestandard MSTP on an MST
port; no explicit configuration is necessary.
Interoperability with Rapid PVST+: Understanding PVST Simulation
MST interoperates with Rapid PVST+ with no need for user configuration. The PVST simulation feature
enables this seamless interoperability.
Note
PVST simulation is enabled by default. That is, by default, all interfaces on the switch interoperate
between MST and Rapid PVST+.
However, you may want to control the connection between MST and Rapid PVST+ to protect against
accidentally connecting an MST-enabled port to a Rapid PVST+-enabled port. Because Rapid PVST+ is
the default STP mode, you may encounter many Rapid PVST+-enabled connections.
Disabling Rapid PVST+ simulation, which can be done per port or globally for the entire switch, moves
the MST-enabled port to the blocking state once it detects it is connected to a Rapid PVST+-enabled port.
This port remains in the inconsistent state until the port stops receiving Rapid PVST+/SSTP BPDUs, and
then the port resumes the normal STP transition process.
Configuring MST
This section includes the following topics:
•
MST Configuration Guidelines, page 9-10
•
Enabling MST, page 9-10
•
Entering MST Configuration Mode, page 9-11