3Com Switch 4200G Configuration Guide

142 CHAPTER 20: MSTP CONFIGURATION
■ To enable the digest snooping feature, the interconnected switches must be
configured with exactly the same MST region-related configuration.
■ The digest snooping feature must be enabled on all the ports of your S4200G
switch that are connected to partners' proprietary protocol-adopted switches in
the same MST region..
■ To change MST region-related configuration, be sure to disable the digest
snooping feature first to prevent possible broadcast storms.
Rapid Transition
Configuration
Introduction Designated ports on switches adopting RSTP or MSTP use the following two types of
packets to implement rapid transition:
■ Proposal packets: Packets sent by designated ports to request rapid transition
■ Agreement packets: Packets used to acknowledge rapid transition requests
Both RSTP and MSTP switches can perform rapid transition operation on a designated
port only when the port receives an agreement packet from the downstream switch.
The difference between RSTP and MSTP switches are:
■ An MSTP upstream switch sends agreement packets to the downstream switch;
and an MSTP downstream switch sends an agreement packet to the upstream
switch only after it receives an agreement packet from the upstream switch.
■ A RSTP upstream switch does not send agreement packets to the downstream
switch.
Figure 39 and Figure 40 illustrate the RSTP and MSTP rapid transition mechanisms.
Figure 39 The RSTP rapid transition mechanism
Designated port
Root port
Upstream switch Downstream switch
Sends proposal packets to
request rapid transition
Sends agreement packets
Root port blocks other
non-
changes to Forwarding
state, and sends
agreement packets
to the upstream switch
Designated port
changes to
Forw arding state
-edge ports
Designated port
Root port
Upstream switch Downstream switch
Sends proposal packets to
request rapid transition
Sends agreement packets
Root port blocks other
non-
changes to Forwarding
state, and sends
agreement packets
to the upstream switch
Designated port
changes to
Forw arding state
-edge ports,
Designated port
Root port
Upstream switch Downstream switch
Sends proposal packets to
request rapid transition
Sends agreement packets
Root port blocks other
non-
changes to Forwarding
state, and sends
agreement packets
to the upstream switch
Designated port
changes to
Forw arding state
-edge ports
Designated port
Root port
Upstream switch Downstream switch
Sends proposal packets to
request rapid transition
Sends agreement packets
Root port blocks other
non-
changes to Forwarding
state, and sends
agreement packets
to the upstream switch
Designated port
changes to
Forw arding state
-edge ports,