Chapter 40 Multiple Spanning Tree Protocol (Mstp) 802.1S; Multiple Spanning Tree Protocol; Multiple Spanning-Tree Regions - Dell PowerConnect B-RX Configuration Manual

Multiple Spanning Tree Protocol (MSTP) 802.1s
802.1s Multiple Spanning Tree Protocol
Multiple Spanning Tree Protocol (MSTP) as defined in IEEE 802.1s allows you to configure multiple
STP instances. This will allow several VLANs to be mapped to a reduced number of spanning-tree
instances. This ensures loop-free topology for 1 or more VLANs that have the same Layer 2
topology.

Multiple spanning-tree regions

Using MSTP, the entire network runs a common instance of RSTP. Within that common instance,
one or more VLANs can be individually configured into distinct regions. The entire network runs the
common spanning tree instance (CST) and the regions run a local instance. The local instance is
known as Internal Spanning Tree (IST). The CST treats each instance of IST as a single bridge.
Consequently, ports are blocked to prevent loops that might occur within an IST and also
throughout the CST. In addition, MSTP can coexist with individual devices running STP or RSTP in
the Common and Internal Spanning Trees instance (CIST). With the exception of the provisions for
multiple instances, MSTP operates exactly like RSTP.
For example, in
entire network is running an instance of CST. Each of the regions is running an instance of IST. In
addition, this network contains Switch 1 running RSTP that is not configured in a region and
consequently is running in the CIST instance. In this configuration, the regions are each regarded
as a single bridge to the rest of the network, as is Switch 1. The CST prevents loops from occurring
across the network. Consequently, a port is blocked at port 1/2 of switch 4.
Additionally, loops must be prevented in each of the IST instances. Within the IST Region 1, a port is
blocked at port 1/2 of switch 4 to prevent a loop in that region. Within Region 2, a port is blocked at
port 3/2 of switch 3 to prevent a loop in that region.
BigIron RX Series Configuration Guide
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Figure 139 a network is configured with two regions, Region 1 and Region 2. The
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