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Catalyst 2928 Switch Software Configuration Guide
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Chapter 17 Configuring MSTP
Configuring MSTP Features
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.
If the switch receives an IEEE 802.1D BPDU after the port migration-delay timer has expired, it
assumes that it is connected to an IEEE 802.1D switch and starts using only IEEE 802.1D BPDUs.
However, if the RSTP switch is using IEEE 802.1D BPDUs on a port and receives an RSTP BPDU
after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port.
Configuring MSTP Features
These sections contain this configuration information:
•
Default MSTP Configuration, page 17-14
•
MSTP Configuration Guidelines, page 17-14
•
Specifying the MST Region Configuration and Enabling MSTP, page 17-15
•
Configuring the Root Switch, page 17-17
(optional)
