112Step Command2. Enter Layer 2 Ethernet or aggregate interfaceview. interface interface-type interface-number3. Perform mCheck. stp mcheckConfiguring Digest SnoopingAs defined in IEEE 802.1s, connected devices are in the same region only when their MST region-relatedconfigurations, including:• Region name.• Revision level.• VLAN-to-instance mappings.A spanning tree device identifies devices in the same MST region by determining the configuration ID inBPDU packets. The configuration ID includes the region name, revision level, and configuration digest. Itis 16-byte long and is the result calculated through the HMAC-MD5 algorithm based onVLAN-to-instance mappings.Because spanning tree implementations vary by vendor, the configuration digests calculated throughprivate keys are different. The devices of different vendors in the same MST region cannot communicatewith each other.To enable communication between an H3C device and a third-party device in the same MST region,enable Digest Snooping on the H3C device port connecting them.Configuration restrictions and guidelinesWhen you configure Digest Snooping, follow these guidelines:• Before you enable Digest Snooping, make sure associated devices of different vendors areconnected and run spanning tree protocols.• With Digest Snooping enabled, in-the-same-region verification does not require comparison ofconfiguration digest, so the VLAN-to-instance mappings must be the same on associated ports.• With Digest Snooping enabled globally, modify the VLAN-to-instance mappings or execute theundo stp region-configuration command to restore the default MST region configuration withcaution. If the local device has different VLAN-to-instance mappings than its neighboring devices,loops or traffic interruption occurs.• To make Digest Snooping take effect, you must enable Digest Snooping both globally and onassociated ports. H3C recommends that you enable Digest Snooping on all associated ports firstand then enable it globally. This will make the configuration take effect on all configured ports andreduce impact on the network.• To prevent loops, do not enable Digest Snooping on MST region edge ports.• H3C recommends that you enable Digest Snooping first and then the spanning tree feature. Toavoid traffic interruption, do not configure Digest Snooping when the network is already workingwell.
