Topology Changes; Configuring Mst - Cisco 7604 Configuration Manual

Chapter 19 Configuring Standard-Compliant IEEE MST

Topology Changes

These are the differences between the RSTP and the 802.1D in handling spanning tree topology changes:
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Configuring MST

These sections describe how to configure MST:
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OL-4266-08
Detection—Unlike 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 router 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—The RSTP does not use TCN BPDUs, unlike 802.1D. However, for 802.1D
interoperability, an RSTP router processes and generates TCN BPDUs.
Acknowledgement—When an RSTP router receives a TCN message on a designated port from an
802.1D router, it replies with an 802.1D configuration BPDU with the TCA bit set. However, if the
TC-while timer (the same as the TC timer in 802.1D) is active on a root port connected to an 802.1D
router and a configuration BPDU with the TCA set is received, the TC-while timer is reset.
This method of operation is only required to support 802.1D routers. The RSTP BPDUs never have
the TCA bit set.
Propagation—When an RSTP router receives a TC message from another router 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 router starts the TC-while timer for all
such ports and flushes the information learned on them.
Protocol migration—For backward compatibility with 802.1D routers, RSTP selectively sends
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 router
processes all BPDUs received on that port and ignores the protocol type.
If the router receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes
that it is connected to an 802.1D router and starts using only 802.1D BPDUs. However, if the RSTP
router is using 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.
Default MST Configuration, page 19-16
MST Configuration Guidelines and Restrictions, page 19-16
Specifying the MST Region Configuration and Enabling MST, page 19-17
Configuring the Root Bridge, page 19-19
Configuring a Secondary Root Bridge, page 19-20
Configuring Port Priority, page 19-21
Configuring Path Cost, page 19-22
Configuring the Switch Priority, page 19-23
Configuring the Hello Time, page 19-24
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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Configuring MST
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