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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter15 Configuring MSTP
Configuring MSTP Features
Specifying the 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 15-7.
By default, the link type is determined 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 con s ide red t o have a share d
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.
Beginning in privileged EXEC mode, follow these steps to override the default link-type s ett ing. Thi s
procedure is optional.
To return the switch to its default setting, use the no spanning-tree link-type interface configuration
command.
Restarting the Protocol Migration Process
A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate
with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with
the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch can also detect
that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (version 3)
associated with a different region, or an RST BPDU (version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D
BPDUs because it cannot determine whether the legacy switch has been removed from the link unless
the legacy switch is the designated switch. A switch also might continue to assi gn a bou nda ry ro l e to a
port when the switch to which it is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switc hes) o n the
switch, use the clear spanning-tree detected-protocols privileged EXEC command.
To restart the protocol migration process on a specific interface, use the clear spanning-tree
detected-protocols interface interface-id privileged EXEC command.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Specify the interface to configure, and enter interface
configuration mode. Valid interfaces include physical ports,
VLANs, and port channels. Valid VLAN IDs are 1 to 1005 when
the SI is installed and 1 to 4094 when the EI is installed. Valid
port-channel numbers are 1 to 6.
Step3 spanning-tree link-type point-to-point Specify that the link type of a port is point-to-point.
Step4 end Return to privileged EXEC mode.
Step5 show spanning-tree mst interface interface-id Verify your entries.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.