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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 21 Configuring MSTP
Information About Configuring MSTP
Secondary Root Switch
When you configure a switch with the extended system ID support as t he secondary root, the switch
priority is modified from the default value (32768) to 28672. The switch is then li kely to become the root
switch for the specified instance if the primary root switch fails. This is assuming that the other network
switches use the default switch priority of 32768 and therefore are unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use
the same network diameter and hello-time values that you used w hen you configured the primary root
switch with the spanning-tree mst instance-id root primary global configuration command.
Port Priority
If a loop occurs, the MSTP uses the port priority when selecting an interface to put into the forwarding
state. You can assign higher priority values (lower numerical values) to interfaces that you want selected
first and lower priority values (higher numerical values) that you want selected last. If all interfaces have
the same priority value, the MSTP puts the interface with the lowest interface number in the forwarding
state and blocks the other interfaces.
Path Cost
The MSTP path cost default value is derived from the media speed of an interface. If a loop occurs, the
MSTP uses cost when selecting an interface to put in the forwarding state. You can assign lower cost
values to interfaces that you want selected first and higher cost values that you want selected last. If all
interfaces have the same cost value, the MSTP puts the interface with the lowest interface number in the
forwarding state and blocks the other interfaces.
Link Type to Ensure Rapid Transitions
If you connect a port to another port through a point-to-point link and the local port become s a
designated port, the RSTP negotiates a rapid transition with the other port by using the
proposal-agreement handshake to ensure a loop-free topology as describe d in the “Rapid Convergence”
section on page 21-9.
By default, the link type is controlled from the duplex mode of the interface: a full-duplex port is
considered to have a point-to-point connection; a half-duplex port i s considered to have a shared
connection. If you have a half-duplex link physically connected point-to-point to a single port on a
remote switch running MSTP, you can override the default setting of the link type and enable rapid
transitions to the forwarding state.
Neighbor Type
A topology could contain both prestandard and IEEE 802 .1s standard compliant devices. By default,
ports can automatically detect prestandard devices, but they can still receive both standard and
prestandard BPDUs. When there is a mismatch between a device a nd its neighbor, only the CIST runs
on the interface.
You can choose to set a port to send only prestandard BPDUs. The prestandard flag a ppears in all the
show commands, even if the port is in STP compatibility mode.