3Com Switch 7750 Configuration Guide Guide

252 CHAPTER 29: MSTP CONFIGURATION
■ To enable the digest snooping feature, the interconnected switches must be
configured with exactly the same MST region-related configurations (including
region name, revision level, and VLAN-to-MSTI mapping).
■ The digest snooping feature must be enabled on all the ports of the Switch
7750 Family that are connected to a partners’ proprietary protocol-adopted
switches in the same MST region.
■ With the digest snooping feature is enabled, the VLAN-to-MSTI mapping
cannot be modified.
■ The digest snooping feature is not applicable on MST region edge ports.
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 57 and Figure 58 illustrate the RSTP and MSTP rapid transition mechanisms.
Figure 57 The RSTP rapid transition mechanism
Designated port
Root port
Upstream sw itch Dow nstream 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 sw itch Dow nstream 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 sw itch Dow nstream 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 sw itch Dow nstream 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,