STP Operation
15-4 Configuring Spanning Tree
Rapid Spanning Tree Operation
Rapid Spanning Tree (RSTP) optimizes convergence in a properly configured network by
significantly reducing the time to reconfigure the network’s active topology when physical
topology or configuration parameter changes occur. RSTP is defined in the IEEE 802.1w standard.
Spanning Tree’s primary goal is to ensure a fully connected, loop-free topology. A secondary goal,
realized with the introduction of RSTP, is to move root and designated ports to the forwarding
state as quickly as possible.
In a stable topology all the root and designated ports will be forwarding and the alternate and
backup ports will be blocking. When there is a network topology change, Spanning Tree
recalculates port roles. Ports which are no longer part of the active topology will be put into
blocking state. New designated ports will only forward after receiving an acknowledgement or, in
the case of a port being connected to a non-RSTP device (802.1d), after a sufficient amount of time
has passed.
When a topology change occurs, a change in port operational status or new information contained
in BPDUs is immediately acted upon. A new root port moves to forwarding state as soon as any
recent former root port is put into blocking state. A designated port moves to forwarding state
once the connected device acknowledges agreement with the new topology information. This is
typically an exchange of two BPDUs. These rules ensure an orderly transition from the old
topology to the new topology by preventing transient loops.
Multiple Spanning Tree Operation
The Multiple Spanning Tree Protocol (MSTP) provides for traffic forwarding on multiple ports for
each bridge. A single Spanning Tree only allows for single root port forwarding per bridge. MSTP
provides for a number of common network requirements that cannot be configured on a single
Spanning Tree (for example, the segregation of traffic over multiple VLANs or optimizing the
utilization of redundant links between switching devices in a network).
An MSTP configuration is made up of one or more:
• Multiple Spanning Tree (MST) Regions – A set of connected bridges that share the same MST
configuration ID
• MST configuration IDs – A unique identifier for each MST region
• Spanning Tree Identifiers (SIDs) – A unique identifier for each Spanning Tree
An MSTP configuration is made up of zero or more Multiple Spanning Tree Instances (MSTIs). An
MSTI is an SID that exists within an MST region other than the default SID 0.
All bridges in the Spanning Tree network are inter-connected by SID 0 and can belong to:
• The Common Spanning Tree (CST) – A Spanning Tree defined in the IEEE 802.1q standard
that assumes one Spanning Tree instance for the entire bridged network, regardless of the
number of VLANs
• An Internal Spanning Tree (IST) instance – A Spanning Tree instance that extends the CST
inside the MST region and represents the entire MST region as a single CST virtual bridge to
the outside world.
One or more MSTs can be part of the Common and Internal Spanning Tree (CIST). The CIST
represents the connectivity of the entire network. Figure 15-3 on page 15-5 provides an overview
of an MST configuration with one MST region within the CIST. The MST region’s configuration ID
name is MSTCentral.
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