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Chapter 18
Configuring MSTP
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 becomes 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 described in the
section on page
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 is 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.
Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This
procedure is optional.
Command
Step 1
configure terminal
Step 2
interface interface-id
Step 3
spanning-tree link-type point-to-point
Step 4
end
Step 5
show spanning-tree mst interface interface-id
Step 6
copy running-config startup-config
To return the port 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 also can 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 detect whether the legacy switch has been removed from the link unless the
legacy switch is the designated switch. A switch also might continue to assign a boundary role 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 switches) on 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.
78-16180-02
18-8.
Purpose
Enter global configuration mode.
Specify an interface to configure, and enter interface
configuration mode. Valid interfaces include physical ports,
VLANs, and port-channel logical interfaces. The VLAN ID
range is 1 to 4094. The port-channel range is 1 to 12.
Specify that the link type of a port is point-to-point.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Catalyst 3750 Switch Software Configuration Guide
Configuring MSTP Features
"Rapid Convergence"
18-23
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Troubleshooting
