Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 331)

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. With the
exception of the provisions for multiple instances, MSTP operates exactly like RSTP.
For example, in Figure 70 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 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.
FIGURE 70 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.
Multiple spanning-tree regions
FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide 331
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