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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter15 Configuring MSTP
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 interop erability 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 informati on (lo wer bri dge ID, lo wer pa th cost, and so fo rth) tha n currently
stored for the port, the RSTP triggers a reconfiguration. If the port i s 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 forw ard- delay ti mer for the por t. Th e 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 messa ge. Th e de signa ted por t
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, high er pa th co st, a nd so fort h tha n
currently stored for the port) with a designated port role, it immediately replies with its own informatio n.
Topology Changes
This section describes the differences between the RSTP and the 802.1D in handling spanning-tre e
topology changes.
•Detection—Unlike 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 chan ge).
State changes on an edge port do not cause a topology chang e. W hen a n RST P swi tch det ects a
topology change, it flushes the learned information on all of its nonedge ports except on those fro m
which it received the TC notification.
•Notification—Unlike 802.1D, which uses TCN BPDUs, the RSTP does not use them. Howev er, 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
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 recei v ed, the TC- while timer
is reset.
This behavior is only required to support 802.1D switches. The RSTP BPDUs never have the TCA
bit set.