H3C S5100-SI Series Operation Manual page 271

Operation Manual – MSTP
H3C S5100-SI/EI Series Ethernet Switches
For RSTP, the upstream switch does not send agreement packets to the
downstream switch.
Figure 1-6
ports in RSTP and MSTP.
Upstream switch
Designated port
changes to
forwarding state
Figure 1-6 The RSTP rapid transition mechanism
Figure 1-7 The MSTP rapid transition mechanism
The cooperation between MSTP and RSTP is limited in the process of rapid transition.
For example, when the upstream switch adopts RSTP, the downstream switch adopts
MSTP and the downstream switch 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 turn to the
forwarding state after a period twice the forward delay.
Some other manufacturers' switches adopt proprietary spanning tree protocols that are
similar to RSTP in the way to implement rapid transition on designated ports. When a
and Figure 1-7 illustrate the rapid transition mechanisms on designated
Proposal for rapid transition
Downstream switch
Root port blocks other non-
edge ports, changes to
forwarding state and sends
Agreement to upstream device
Root port
Designated port
1-50
Chapter 1 MSTP Configuration