Figure 92 – Brocade TurboIron 24X Series Configuration Guide User Manual
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Brocade TurboIron 24X Series Configuration Guide
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802.1s Multiple Spanning Tree Protocol
a network is configured with two regions: Region 1 and Region 2. The
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 MSTP 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 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 2 of switch 4 to prevent a loop in that region. Within Region 2, a port is blocked at
port 2 of switch 3 to prevent a loop in that region.
FIGURE 92
MSTP configured network
The following definitions describe the STP instances that define an MSTP configuration.
Common Spanning (CST) – CST is defined in 802.1q and assumes one spanning-tree instance
for the entire bridged network regardless of the number of VLANs. In MSTP, an MSTP region
appears as a virtual bridge that runs CST.
Internal Spanning Tree (IST) – IST is a new terminology introduced in 802.1s. An MSTP bridge
must handle at least these two instances: one IST and one or more MSTIs (Multiple Spanning
Tree Instances). Within each MST region, the MSTP maintains multiple spanning-tree
instances. Instance 0 is a special instance known as IST, which extends CST inside the MST
region. IST always exists if the switch runs MSTP. Besides IST, this implementation supports up
to 15 MSTIs, numbered from 1 to 4094.
Common and Internal Spanning Trees (CIST) – CIST is a collection of the ISTs in each MST
region and the CST that interconnects the MST regions and single spanning trees.
Multiple Spanning Tree Instance (MSTI) – The MSTI is identified by an MST identifier (MSTid)
value between 1 and 4094.
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