Configuring Stp; Understanding Basic Stp Features; Supported Stp Instances; Stp Overview - Cisco Catalyst 2950 Software Manual

Understanding Basic STP Features

For information about advanced STP features, see the
on page 9-9

Supported STP Instances

This software release supports the per-VLAN spanning tree (PVST) and a maximum of 64 spanning-tree
instances. If more VLANs are defined in the VLAN Trunking Protocol (VTP) than STP instances, you
can enable STP on only 64 VLANs. The remaining VLANs operate with STP disabled.
If 64 instances of STP are already in use, you can disable STP on one of the VLANs and then enable it
on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global configuration
command to disable STP on a specific VLAN, and use the spanning-tree vlan vlan-id global
configuration command to enable STP on the desired VLAN.
Caution Switches that are not running STP still forward Bridge Protocol Data Units (BPDUs) that they receive
so that the other switches on the VLAN that have a running spanning-tree instance can break loops.
Therefore, STP must be running on enough switches to break all the loops in the network; for example,
at least one switch on each loop in the VLAN must be running STP. It is not absolutely necessary to run
STP on all switches in the VLAN; however, if you are running STP only on a minimal set of switches,
an incautious change to the network that introduces another loop into the VLAN can result in a broadcast
storm.
Note If you have already used all available spanning-tree instances on your switch, adding another VLAN
anywhere in the VTP domain creates a VLAN that is not running STP on that switch. If you have the
default allowed list on the trunk ports of that switch, the new VLAN is carried on all trunk ports.
Depending on the topology of the network, this could create a loop in the new VLAN that will not be
broken, particularly if there are several adjacent switches that have all run out of spanning-tree instances.
You can prevent this possibility by setting allowed lists on the trunk ports of switches that have used up
their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many cases and
can make it more labor-intensive to add another VLAN to the network.
Spanning-tree commands determine the configuration of VLAN spanning-tree instances. You create a
spanning-tree instance when you assign an interface to a VLAN. The spanning-tree instance is removed
when the last interface is moved to another VLAN. You can configure switch and port parameters before
an spanning-tree instance is created; these parameters are applied when the spanning-tree instance is
created.

STP Overview

STP is a link management protocol that provides path redundancy while preventing undesirable loops in
the network. For an Ethernet network to function properly, only one active path can exist between any
two stations. STP 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.
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 network.
Switches send and receive STP frames at regular intervals. The switches do not forward these frames,
but use the frames to construct a loop-free path.
Catalyst 2950 Desktop Switch Software Configuration Guide
9-2
and the "Configuring Advanced STP Features" section on page
Chapter 9
"Understanding Advanced STP Features" section
9-30.

Configuring STP

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