3Com Switch 8807, Configuration Manual

Page: 155 / 883

18
D
IGEST
S
NOOPING
C
ONFIGURATION
Introduction to Digest
Snooping
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.
Digest Snooping
Configuration
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.