3Com 4800G Series, Configuration Manual

Page: 231 / 1246

Configuring No Agreement Check 231
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 differences between RSTP and MSTP switches are:
■
For MSTP, the downstream device’s root port sends an agreement packet only
after it receives an agreement packet from the upstream device.
■
For RSTP, the down stream device sends an agreement packet regardless of
whether an agreement packet from the upstream device is received.
Figure 59 and Figure 60 show the rapid state transition mechanism on MSTP and
RSTP designated ports.
Figure 59 Rapid state transition of a designated port in MSTP
Figure 60 Rapid state transition of a designated port in RSTP
If the upstream device comes from another vendor, the rapid state transition
implementation may be limited. For example, when the upstream device adopts
RSTP, the downstream device adopts MSTP and does not support RSTP mode, the
root port on the downstream device receives no agreement packet from the
upstream device and thus sends no agreement packets to the upstream device. 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.
In this case, you can enable the No Agreement Check feature on the downstream
device’s port to perform rapid state transition.
Root port
Designated port
Root port blocks other
non-edge ports
Root port changes to
forwarding state and
sends Agreement
Downstream switch Upstream Switch
Agreement
Proposal for rapid transition
Designated port
changes to
forwarding state
Agre
emen
t
Root port
Designated port
Root port blocks other
non-edge ports , changes
to forwarding state and
sends Agreement to
upstream switch
Downstream switch Upstream Switch
Proposal for rapid transition
Designated port
changes to
forwarding state
Agree
ment