3Com Switch 7750 Configuration Guide Guide

Rapid Transition Configuration 253
Figure 58 The MSTP rapid transition mechanism
Limitation on the combination of RSTP and MSTP exists to implement rapid
transition. For example, when the upstream switch adopts RSTP, the downstream
switch adopts MSTP and does not support RSTP-compatible mode, the root port
on the downstream switch receives no agreement packet from the upstream
switch and thus sends no agreement packets to the upstream switch. As a result,
the designated port of the upstream switch fails to transit rapidly and can only
change to the Forwarding state after a period twice the Forward Delay.
Some partners’ switches adopt proprietary spanning tree protocols that are similar
to RSTP in the way to implement rapid transition on designated ports. When a
switch of this kind, operating as the upstream switch, connects with the Switch
7750 running MSTP, the upstream designated port fails to change their states
rapidly.
The rapid transition feature is developed to resolve this problem. When Switch
7750 running MSTP is connected in the upstream direction to a partner’s switch
running proprietary spanning tree protocol, you can enable the rapid transition
feature on the ports of the Switch 7750 operating as the downstream switch.
Among these ports, those operating as the root ports will then send agreement
packets to their upstream ports after they receive proposal packets from the
upstream designated ports, instead of waiting for agreement packets from the
upstream switch. This enables designated ports of the upstream switch to change
their states rapidly.
Rapid Transition
Configuration
Prerequisites
As shown in Figure 59, Switch 7750 is connected to a partner’s switch. The former
operates as the downstream switch, and the latter operates as the upstream
switch. The network operates normally.
The upstream switch is running a proprietary spanning tree protocol that is similar
to RSTP in the way to implement rapid transition on designated ports. Port 1 is a
designated port.
The downstream switch is running MSTP. Port 2 is the root port.
Designated port
Root port
Upstream sw itch Dow nstream switch
Send proposal packets
to request rapid transition
Send agreement packets
Root port changes to
Forw arding state and
sends agreement packets
to upstream switchDesignated port
change to
Forw arding state
Send agreement packets
Root port blocks
other non-edge ports
-
Designated port
Root port
Upstream sw itch Dow nstream switch
Send proposal packets
to request rapid transition
Send agreement packets
Root port changes to
Forw arding state and
sends agreement packets
to upstream switchDesignated port
change to
Forw arding state
Send agreement packets
Root port blocks
other non-edge ports
-