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1170 FastIron Configuration Guide

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802.1s Multiple Spanning Tree Protocol

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 156 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 156 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.

BigIron

Switch 1

Switch 2

Switch 3

Switch 4

Switch 5

Switch 6

Switch 2

Switch 3

Switch 4

Switch 5

Region 1

Region 2

Port2/1

Port2/2

Port2/3

Port2/1

Port1/2

Port1/1

Port1/3

Port1/4

Port1/2

Port1/1

Port1/2

Port2/2

Port1/1

Port1/4

Port1/5

Port1/3

Port1/2

Port3/1

Port3/3

Port3/2

Port3/1

Port3/2