FunctionBit
Agreement6
Topology change acknowledgement (TCA)7
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 proposal. The port 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 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 IEEE 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 (such as a higher switch ID or a higher path cost 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.
• 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 deletes
the learned information on all of its nonedge ports except on those from which it received the TC
notification.
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
OL-29424-01 57
Configuring Multiple Spanning-Tree Protocol
RSTP Overview
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