Cisco IE 4000, Software Configuration Manual

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Configuring MSTP
Information About Configuring MSTP
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 (higher switch 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.

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.

Notification—Unlike IEEE 802.1D, which uses TCN BPDUs, the RSTP does not use them. However, for IEEE 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 IEEE 802.1D configuration BPDU with the TCA bit set. However, if the TC-while timer (the
same as the topology-change timer in IEEE 802.1D) is active on a root port connected to an IEEE 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.

Propagation—When an RSTP switch receives a TC message from another switch through a designated or root port,
it propagates the change to all of its nonedge, designated ports and to the root port (excluding the port on which it
is received). The switch starts the TC-while timer for all such ports and flushes the information learned on them.

Protocol migration—For backward compatibility with IEEE 802.1D switches, RSTP selectively sends IEEE 802.1D
configuration BPDUs and TCN BPDUs on a per-port basis.
When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which RSTP BPDUs
are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port
and ignores the protocol type.