CHAPTER
13-1
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
13
Understanding and Configuring STP
This chapter describes how to configure the Spanning Tree Protocol (STP) on a Catalyst 4500 series
switch. It also provides guidelines, procedures, and configuration examples.
This chapter includes the following major sections:
Overview of STP, page 13-1
Default STP Configuration, page 13-6
Configuring STP, page 13-7
Note For information on configuring the PortFast, UplinkFast, and BackboneFast, and other spanning tree
enhancements, see Chapter 14, “Configuring STP Features.”
Note For complete syntax and usage information for the switch commands used in this chapter, refer to the
Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm.

Overview of STP

STP is a Layer 2 link management protocol that provides path redundancy whi le preventing undesirable
loops in the network. For a Layer 2 Ethernet network to function properly, only one active path can exist
between any two stations. A loop-free subset of a network topology is called a spanning tree. The
operation of a spanning tree is transparent to end stat ions, which cannot detect whether they are
connected to a single LAN segment or a switched LAN of multiple segments.
A Catalyst 4500 series switch use STP (the IEEE 802.1D bridge protocol) on all VLANs. By default, a
single spanning tree runs on each configured VLAN (provided you do not manually disable the spanning
tree). You can enable and disable a spanning tree on a per-VLAN basis.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning tree algorithm calculates the best loop-free path throughout a switched Layer 2
network. Switches send and receive spanning tree frames at regular intervals. The switches do not
forward these frames, but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network,
end stations might receive duplicate messages and switches might learn end station MAC addresses on
multiple Layer 2 interfaces. These conditions result in an unstable network.