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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter15 Configuring MSTP
Configuring MSTP Features
Specifying the Link Type to Ensure Rapid Transitions
If you connect an NNI to another NNI through a poi nt-to -poi nt lin k an d t he lo cal po rt bec ome 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 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 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 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 dete ct
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 switc hes) o n the
switch, use the clear spanning-tree detected-protocols privileged EXEC command.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Specify an interface to configure, and enter interface
configuration mode. Valid interfaces include physical NNIs,
VLANs, and NNI port-channel logical interfaces. The VLAN ID
range is 1 to 4094. The port-channel range is 1 to 48.
Note If a physical interface is not an NNI, enter the port-type
nni interface configuration command before you
configure the link type.
If the interface is a port channel, all members of the port
channel must be NNIs.
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.