3Com Switch 7754, Configuration Manual

Page: 270 / 940

270 C
HAPTER
30: MSTP C
ONFIGURATION
■
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 62 and Figure 63 illustrate the RSTP and MSTP rapid transition mechanisms.
Figure 62 The RSTP rapid transition mechanism
Figure 63 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
Root port blocks other non-
edge ports , changes to
forwarding state and sends
Agreement to upstream devic
Downstream switch
Upstream switch
Proposal for rapid transition
Agre
eme
nt
Designated port
hanges to
orwarding state
Root port
Designated port
Downstream switch
Root port blocks other
non-edge ports
Root port changes to
forwarding state and
sends Agreement to
upstream switch
Designated port
changes to
forwarding state
Upstream switch
Proposal for rapid transition
Agreement
Ag re
em en
t