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Understanding RSTP
The RSTP does not have a separate topology change notification (TCN) BPDU. It uses the topology
change (TC) flag to show the topology changes. However, for interoperability with 802.1D switches, the
RSTP switch processes and generates TCN BPDUs.
The learning and forwarding flags are set according to the state of the sending port.
Processing Superior BPDU Information
If a port receives superior root information (lower bridge ID, lower path cost, and so forth) than currently
stored for the port, the RSTP triggers a reconfiguration. If the port is proposed and is selected as the new
root port, RSTP forces all the other ports to synchronize.
If the BPDU received is an RSTP BPDU with the proposal flag set, the switch sends an agreement
message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the switch does
not set the proposal flag and starts the forward-delay timer for the port. The new root port requires twice
the forward-delay time to transition to the forwarding state.
If the superior information received on the port causes the port to become a backup or alternate port,
RSTP sets the port to the blocking state but does not send the agreement message. The designated port
continues sending BPDUs with the proposal flag set until the forward-delay timer expires, at which time
the port transitions to the forwarding state.
Processing Inferior BPDU Information
If a designated port receives an inferior BPDU (higher bridge ID, higher path cost, and so forth than
currently stored for the port) with a designated port role, it immediately replies with its own information.
Topology Changes
This section describes the differences between the RSTP and the IEEE 802.1D in handling spanning-tree
topology changes.
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Detection—Unlike IEEE 802.1D in which any transition between the blocking and the forwarding
state causes a topology change, only transitions from the blocking to the forwarding state cause a
topology change with RSTP (only an increase in connectivity is considered a topology change).
State changes on an edge port do not cause a topology change. When an RSTP switch detects a
topology change, it flushes the learned information on all of its nonedge ports except on those from
which it received the TC notification.
Notification—Unlike IEEE 802.1D, which uses TCN BPDUs, the RSTP does not use them.
However, for 802.1D interoperability, an RSTP switch processes and generates TCN BPDUs.
Acknowledgement—When an RSTP switch receives a TCN message on a designated port from an
IEEE 802.1D switch, it replies with an 802.1D configuration BPDU with the TCA bit set. However,
if the TC-while timer (the same as the topology-change timer in 802.1D) is active on a root port
connected to an 802.1D switch and a configuration BPDU with the TCA bit set is received, the
TC-while timer is reset.
This behavior is only required to support IEEE 802.1D switches. The RSTP BPDUs never have the
TCA bit set.
Chapter 15
Configuring MSTP
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