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Performing mCheck
If a port on a device that is running MSTP, RSTP, or PVST connects to an STP device, this port
automatically transitions to the STP mode. However, it cannot automatically transition back to the
original mode under the following circumstances:
•
The STP device is shut down or removed.
•
The STP device transitions to the MSTP, RSTP, or PVST mode.
Suppose Device A running STP, Device B with no spanning tree feature enabled, and Device C
running MSTP, RSTP, or PVST are connected in order. Device B will transparently transmit the STP
BPDUs, and the port on Device C and connecting to Device B will transition to the STP mode. After
you enable the spanning tree feature on Device B, to run MSTP, RSTP, or PVST between Device B
and Device C, you must perform an mCheck operation on the ports interconnecting Device B and
Device C, in addition to configuring the spanning tree to operate in MSTP, RSTP, or PVST mode on
Device B.
To forcibly transition the port to operate in the original mode, you can perform an mCheck operation.
The following methods for performing mCheck produce the same result.
Performing mCheck globally
Step Command
1.
Enter system view.
system-view
2.
Perform mCheck.
stp mcheck
Performing mCheck in interface view
Step Command
1.
Enter system view.
system-view
2.
Enter Layer 2 Ethernet interface view or Layer 2
aggregate interface view.
interface interface-type interface-number
3.
Perform mCheck.
stp mcheck
NOTE:
An mCheck operation takes effect on a device that operates in MSTP, RSTP, or PVST mode.
Configuring Digest Snooping
As defined in IEEE 802.1s, connected devices are in the same region only when their MST
region-related configurations (region name, revision level, and VLAN-to-instance mappings) are
identical. A spanning tree device identifies devices in the same MST region by determining the
configuration ID in BPDU packets. The configuration ID includes the region name, revision level, and
configuration digest, which is in 16-byte length and is the result calculated via the HMAC-MD5
algorithm based on VLAN-to-instance mappings.
Spanning tree implementations vary with vendors, and the configuration digests calculated using
private keys is different, so devices of different vendors in the same MST region cannot
communicate with each other.
