Chapter 14 Understanding And Configuring Stp; Overview Of Stp - Cisco 4500M Software Manual

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:
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Note For information on configuring the PortFast, UplinkFast, and BackboneFast, and other spanning tree
enhancements, see
For complete syntax and usage information for the switch commands used in this chapter, refer to the
Note
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 while 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 stations, 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.
OL-6696-01
Overview of STP, page 14-1
Default STP Configuration, page 14-6
Configuring STP, page 14-7
Chapter 15, "Configuring STP Features."
C H A P T E R
Software Configuration Guide—Release 12.2(25)EW
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