Configuring Fast Transition - 3Com 8807 Configuration Manual

160 C 19: F
HAPTER AST
Configuring Fast
transition
Configuration
Preparation
T
RANSITION
Figure 34 Designated port fast transition mechanism of MSTP
Upstream switch Upstream switch
The designated The designated
port enters the port enters the
forwarding state forwarding state
There is a certain limit on fast transition cooperation of RSTP and MSTP. For
example, the upstream switch runs RSTP and the downstream switch runs MSTP,
and the downstream MSTP does not support the compatible RSTP mode. As the
root port of the downstream MSTP-enabled switch cannot receive the agreement
packet from the upstream switch, the downstream switch does not send the
agreement packet to the upstream switch. As a result, the designated port of the
upstream switch cannot implement fast transition. Instead, the state becomes
forwarding state after double forward delay.
In practice, since private protocols related to STP are configured on the switch of
another vendor and the designated port state transition mechanism is similar to
that of RSTP, the designated ports cannot implement fast state transition when
these switches serving as upstream switches interconnect with the Switch 8800
Family series routing switch that runs MSTP.
To avoid such problem, Switch 8800 Family series routing switches provide fast
transition. When the Switch 8800 Family series routing switch interconnects with
the switches from another vendor, you can enable fast transition on the port of
the S5800 switch that serves as the downstream switch. If this port is a root port,
after receiving the proposal packet from the designated port of the upstream
switch, the port sends the agreement packet to the upstream switch initiatively
rather than sends the agreement packet after receiving the agreement packet. As
a result, the upstream switch can perform fast state transition on the designated
port.
The Switch 8800 Family series routing switch serves as the downstream switch and
the switch from another vendor serves as the upstream switch. They have been
connected correctly, as shown in Figure 35.
A private protocol related to STP is enabled on the upstream switch. The
designated port state transition mechanism is similar to that of RSTP. Port 1 is a
designated port.
MSTP is enabled on the downstream Switch 8800 Family switch. Port 2 is a
designated port.
Send the proposal message to Send the proposal message to
request for fast transition request for fast transition
Send the agreement message Send the agreement message
Send the agreement message Send the agreement message Send the agreement message Send the agreement message
Downstream switch Downstream switch
The root port congests other The root port congests other
non-edge ports. non-edge ports.
The root port enters the The root port enters the
forwarding state, and then forwarding state, and then
Forwards the agreement Forwards the agreement
message to the upstream switch message to the upstream switch
Root port Root port
Designated port Designated port