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Configuring STP
Prerequisites for Configuring STP
When you configure VTP, you must configure a trunk port so that the switch can send and receive VTP advertisements
to and from other switches in the domain.
For more information, see Configuring an Ethernet Interface as a Trunk Port, page 286.
Restrictions for Configuring STP
If you are configuring VTP on a cluster member switch to a VLAN, use the rcommand privileged EXEC command to
log in to the member switch.
In VTP versions 1 and 2, when you configure extended-range VLANs on the switch, the switch must be in VTP
transparent mode. VTP version 3 also supports creating extended-range VLANs in client or server mode.
Information About Configuring STP
This chapter describes how to configure the Spanning Tree Protocol (STP) on port-based VLANs on the switch. The
switch can use either the per-VLAN spanning-tree plus (PVST+) protocol based on the IEEE 802.1D standard and Cisco
proprietary extensions, or the rapid per-VLAN spanning-tree plus (rapid-PVST+) protocol based on the IEEE 802.1w
standard.
STP
STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the network. For a
Layer 2 Ethernet network to function properly, only one active path can exist between any two stations. Multiple active
paths among end stations cause loops in the network. If a loop exists in the network, end stations might receive duplicate
messages. Switches might also learn end-station MAC addresses on multiple Layer 2 interfaces. These conditions result
in an unstable network. Spanning-tree operation is transparent to end stations, which cannot detect whether they are
connected to a single LAN segment or a switched LAN of multiple segments.
The STP uses a spanning-tree algorithm to select one switch of a redundantly connected network as the root of the
spanning tree. The algorithm calculates the best loop-free path through a switched Layer 2 network by assigning a role
to each port based on the role of the port in the active topology:
Root—A forwarding port elected for the spanning-tree topology
Designated—A forwarding port elected for every switched LAN segment
Alternate—A blocked port providing an alternate path to the root bridge in the spanning tree
Backup—A blocked port in a loopback configuration
The switch that has all of its ports as the designated role or as the backup role is the root switch. The switch that has at
least one of its ports in the designated role is called the designated switch.
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