3com 8807 User Guide

According to IEEE 802.1s, two connected switches can communicate with each 
other through multiple spanning tree instances (MSTIs) in a multiple spanning tree 
protocol (MSTP) region only when they are configured with the same region 
settings. With MSTP employed, interconnected switches determine whether or not 
they are in the same region by checking the configuration IDs of the bridge 
protocol data units (BPDUs) between them. (A configuration ID comprises 
information such as region ID, configuration digest.)

As switches of some manufacturers come with some proprietary protocols 
concerning spanning trees employed, a switch of this type cannot communicate 
with other switches in an MSTP region even if it is configured with the same MSTP 
region settings as other switches in the MSTP region.

This kind of problems can be overcome by implementing digest snooping. Digest 
snooping enables a switch to track and maintain configuration digests of other 
switches that are in the same region and come from other manufacturers by 
examining their BPDUs. It also enables the switch to insert corresponding 
configuration digests in its BPDUs destined for these switches. In this way, 
switches of different manufacturers are capable of communicating with each 
other in an MSTP region.

Note that:

1 When implementing digest snooping in an MSTP region, make sure that the 

region configurations of the switches of different manufacturers are exactly the 
same to prevent possible broadcast storm caused by otherwise inconsistent 
mapping relationships between VLANs and VPN instances of each switch.

2 If you want to change the configuration of a region with one or multiple of its 

switches being digest snooping-enabled, be sure to disable digest snooping on 
these switches first to prevent possible broadcast storm caused by otherwise 
inconsistent mapping relationships between VLANs and VPN instances of each 
switch.

3 A digest snooping-enabled switch always keeps the latest configuration digests it 

receives. A configuration digest remains valid even if the corresponding port goes 
down.

Configure digest snooping on a switch to enable it to communicate in MSTP 
regions through MSTI with other switches that are configured with some 
proprietary protocols to calculate configuration digest.