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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP Understanding RSTP
The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch
on that LAN. The port role in the proposal message is always set to the designated port.
The sending switch sets the agreement flag in the RSTP BPDU to accept the previous pro posal. The p ort
role in the agreement message is always set to the root port.
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 IEEE 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 switch ID, lower path cost, and so forth) than 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 IEE E 802.1 D BPDU, the s witch
does not set the proposal flag and starts the forward-delay timer for the p ort. 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 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 switch ID, h ighe r pat h cost , a nd so for t h th an
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 handl ing spanning-tree
topology changes.
•Detection—Unlike IEEE 802.1D in which any transition between the blocking and the forwarding
state causes a topology change, only transitions from the bloc king t o th e for warding state cause a
topology change with RSTP (only an increase in connect ivity i s c onsid er ed a t op ology ch an 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 deletes the learned information on all of its nonedge por ts except on those from
which it received the TC notification.
5Forwarding
6 Agreement
7 Topology change acknowledgement (TCA)
Table18-3 RSTP BPDU Flags (continued)
Bit Function