3Com 5500-EI PWR Reference Manual page 247

As some other manufacturers' switches adopt proprietary spanning tree protocols, they cannot
interwork with other switches in an MST region even if they are configured with the same MST
region-related settings as other switches in the MST region.
This kind of problem can be overcome by implementing the digest snooping feature. If a switch port is
connected to another manufacturer's switch that has the same MST region-related settings but adopts
a proprietary spanning tree protocol, you can enable the digest snooping feature on the port that will be
receiving BPDU packets from another manufacturer's switch. Then the switch considers these BPDU
packets to be from its own MST region and records the configuration digests carried in the BPDU
packets received from the switch, which will be put in the BPDU packets to be sent to another
manufacturer's switch. In this way, the switch can interwork with another manufacturer's switches in an
MST region.
When the digest snooping feature is enabled on a port, the port turns to the discarding state. That is,
the port stops sending BPDU packets. The port is not involved in the STP calculation until it
receives BPDU packets from the peer port.
The digest snooping feature is needed only when your switch is connected to another
manufacturer's switches adopting proprietary spanning tree protocols.
To enable the digest snooping feature, the interconnected switches and another manufacturer's
switch adopting proprietary spanning tree protocols must be configured with exactly the same MST
region-related configurations (including region name, revision level, and VLAN-to-instance
mapping).
The digest snooping feature must be enabled on all the switch ports that connect to another
manufacturer's switches adopting proprietary spanning tree protocols in the same MST region.
When the digest snooping feature is enabled globally, the VLAN-to-instance mapping table cannot
be modified.
The digest snooping feature is not applicable to boundary ports in an MST region.
The digest snooping function is not applicable to edge ports in an MST region.
Examples
# Enable the digest snooping feature on Ethernet 1/0/1.
Enable the digest snooping feature on Ethernet 1/0/1 in Ethernet port view.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] interface Ethernet 1/0/1
[Sysname-Ethernet1/0/1] stp config-digest-snooping
[Sysname-Ethernet1/0/1] quit
[Sysname] stp config-digest-snooping
Enable the digest snooping feature on Ethernet 1/0/1 in system view.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] stp interface Ethernet 1/0/1 config-digest-snooping
[Sysname] stp config-digest-snooping
1-19