R2511-HP MSR Router Series Layer 2 - LAN Switching Configuration Guide(V5)

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Performing mCheck
If a port on a device that is running MSTP or RSTP connects to an STP device, this port automatically
migrates to STP mode. However, it cannot automatically transit back to the original mode in the following
situations:
• The STP device is shut down or removed.
• The STP device transits to MSTP or RSTP mode.
Suppose Device A running STP, Device B with no spanning tree feature enabled, and Device C running
RSTP or MSTP are connected in order. Device B will transparently transmit STP BPDUs, and the port on
Device C connecting to Device B will transit to the STP mode. After you enable the spanning tree feature
on Device B, to run RSTP or MSTP 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 RSTP or MSTP mode on Device B.
To forcibly transit the port to operate in the original mode, you can perform an mCheck operation. An
mCheck operation takes effect on a device that operates in MSTP or RSTP mode.
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 Ethernet interface view or Layer
2 aggregate interface view.
interface interface-type interface-number
3. Perform mCheck.
stp mcheck
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 by using the HMAC-MD5 algorithm based on
VLAN-to-instance mappings.
Because spanning tree implementations vary with vendors, the configuration digests calculated by using
private keys are different. Devices of different vendors in the same MST region cannot communicate with
each other.