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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter13 Understanding and Conf iguring STP
Configuring STP
The following example shows how to verify the configuration:
Switch# show spanning-tree summary totals
Switch is in rapid-pvst mode
Root bridge for:VLAN0001
Extended system ID is disabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
1 vlan 0 0 0 2 2
Switch#
Specifying the Link Type
Rapid connectivity is established only on point-to-point links. Spanning tree views a point-to-point link
as a segment connecting only two switches running the spanning tree algorithm. Because the switch
assumes that all full-duplex links are point-to-point links and that half-duplex lin ks are shared links, you
can avoid explicitly configuring the link type. To configure a specific link type, use the spanning-tree
linktype command.
Restarting Protocol Migration
A switch running both MSTP and RSTP supports a built-in protocol migration process that enables the
switch 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.
Furthermore, when an MSTP switch receives a legacy BPDU, it can also detect the following:
•that a port is at the boundary of a region
•an MST BPDU (version 3) associated with a different region, or
•an RST BPDU (version 2).
The switch, however, does not automatically revert to the MSTP mode if it no lo nger receives 802.1D
BPDUs because it cannot determine whether or not the legacy switch has been removed from the link
unless the legacy switch is the designated switch. A switch also might contin ue 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 on the entire switch (that is, to force r enegotiation with
neighboring switches), use the clear spanning-tree detected-protocols commands in privileged EXEC
mode. To restart the protocol migration process on a specific interface, enter the clear spanning-tree
detected-protocols interface command in interface-id privileged EXEC mode.