Specifications

10-13
Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 10 Configuring MSTP
Configuring MSTP Features
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 flushes 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 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 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 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’s 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
• Default MSTP Configuration, page 10-14
• MSTP Configuration Guidelines, page 10-14
• Specifying the MST Region Configuration and Enabling MSTP, page 10-15 (required)
• Configuring the Router as a Root Switch, page 10-16 (optional)
• Configuring a Secondary Root Switch, page 10-18 (optional)
• Configuring Port Priority, page 10-19 (optional)
• Configuring Path Cost, page 10-20 (optional)
• Configuring the Device Priority, page 10-21 (optional)
• Configuring the Hello Time, page 10-22 (optional)