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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP
Configuring RSTP and MSTP Featur es

Specifying the Link Type to Ensure Rapid Transitions

If you con nect a port to anothe r port throug h a point-t o-po int link an d the local port beco mes 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 “Rapid Convergence”
section on page 12-3.
By default, the link type is determined from the duplex mode of the interface: a full-duplex port is
conside red to have a po int -to- point co nne ctio n; a ha lf- dup lex por t i s con s ide red t o have a share d
connec tion. If you have a half- duplex link physic ally co nnected poi nt-to-p oint to a single port on a
remote swit ch runn ing RSTP, you can override th e default sett ing of the link type and enabl e rapid
transitions to the forwarding state.
Beginning i n privileged EX EC mode , foll ow these s te ps t o 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 suppor ts a built-in prot ocol 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 de tect th at a por t is at the boun dary of a re gio n when it rece i v es a le ga cy BPDU , an MST
BPDU (version 3) as sociat ed with a di fferent region, or an RST BPDU (version 2).
However, the switch does not automatically revert to th e MSTP m ode if it no longer receives 802.1D
BPDUs becau se it cannot determi ne whether th e legacy switch has been removed from the link unless
the legacy switch i s the desi gnat ed sw itch. A sw it ch a lso m ight c on tin ue to assign a bou ndary ro le to a
port when the switch to which it is connected has joined the region.
To restart t he pr otoc ol m igra tion pr ocess (f orc e the r enegotiat ion with ne ighb orin g sw itc hes) o n the
entire switc h, you can use the clear spanni ng-tre e dete cted-prot ocols pri vile ged E XEC co mmand. Us e
the clear spanning-tree detected-protocols interface interface-id privile ged EXEC command to restart
the prot ocol mi grat ion proc ess on a spe ci fic interface .
Command Purpose
Step 1 configure terminal Enter g lob al c onfigura tion m ode .
Step 1 interface interface-id Enter inte rface config urati on m ode, and spe cify the inter f a ce 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.
Step 2 spanning-tree link-type point-to-point Specify that the link type of a port is point-to-point.
Step 3 end Return to pr ivileged EX EC m ode.
Step 4 show spanning-tree mst interface interface-id Verify your entries.
Step 5 copy running-config startup-config (Optional) Save your entries in the configuration file.