Scaling the Spanning Tree Protocol
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IEEE 802.1q standard, Cisco's existing PVST has been modified with addi-
tional features enabling it to support a link across the IEEE 802.1q Common
Spanning Tree region.
PVST+ includes features such as the following:
Provides notification of inconsistencies related to port trunking or
VLAN identification across the switches.
Adds mechanisms to ensure that there is no unknown configuration.
Tunnels PVST BPDUs through the 802.1q VLAN region as multi-
cast data.
Provides compatibility with IEEE 802.1q's CST and Cisco's PVST
protocols.
Interoperates with 802.1q-compliant switches using CST through
802.1q trunking. A CST BPDU is transmitted or received with an IEEE
standard bridge group MAC address.
Blocks ports that receive inconsistent BPDUs in order to prevent for-
warding loops.
Notifies users via syslog messages about all inconsistencies.
Scaling the Spanning Tree Protocol
T
he STP prevents loops in layer 2 switched networks and is basically
plug-and-play. However, it may be advantageous to change some of the
default timers and settings to create a more stable environment.
In this section, we'll discuss how to scale the STP protocol on a large,
switched internetwork. It is important to understand how to provide proper
placement of the root bridge to create an optimal topology. If the root bridge
is automatically chosen through an election, which is the default, the actual
path the frames may take might not be the most efficient. The administrator
can then change the root placement, which allows for a possibly more opti-
mal path. However, you can cause more damage as well, but hopefully you'll
have thought out your network design before making any changes.
To change the root placement, you need to do the following:
Determine the root device.
Configure the device.
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