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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter11 Configuring RSTP and MSTP
Configuring RSTP and 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 11-3.
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 RSTP, 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:
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 both MSTP and RSTP 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 re gi on wh en i t rece i v es a l eg ac y BP DU, a n 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
entire switch, you can use the clear spanning-tree detected-protocols p ri vile ged EXEC comm and. Use
the clear spanning-tree detected-protocols int erface interface-id pri vileged EXEC command to restart
the protocol migration process on a specific interface.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step1 interface interface-id Enter interface configuration mode, and specify the interface to
configure. Valid interfaces include physical ports, VLANs, and
port channels. Valid VLAN IDs are 1 to 4094; valid port-channel
numbers are 1 to 6.
Step2 spanning-tree link-type point-to-point Specify that the link type of a port is point-to-point.
Step3 end Return to privileged EXEC mode.
Step4 show spanning-tree mst interface interface-id Verify your entries.
Step5 copy running-config startup-config (Optional) Save your entries in the configuration file.