Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 332)

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.
MSTP Region - These are clusters of bridges that run multiple instances of the MSTP protocol.
Multiple bridges detect that they are in the same region by exchanging their configuration (instance to
VLAN mapping), name, and revision-level. Therefore, if you need to have two bridges in the same
region, the two bridges must have identical configurations, names, and revision-levels. Also, one or
more VLANs can be mapped to one MSTP instance (IST or MSTI) but a VLAN cannot be mapped to
multiple MSTP instances.
NOTE
One or more VLANs can be mapped to one MSTP instance (IST or MSTI) but a VLAN cannot be
mapped to multiple MSTP instances.
Configuration notes
When configuring MSTP, note the following:
• With MSTP running, enabling static trunk on ports that are members of many VLANs (4000 or more
VLANs) will keep the system busy for 20 to 25 seconds.
Configuring MSTP mode and scope
With the introduction of MSTP, a system can be either under MSTP mode or not under MSTP mode.
The default state is to not be under MSTP mode. MSTP configuration can only be performed in a
system under MSTP mode.
With a system configured under MSTP mode, there is a concept called MSTP scope. MSTP scope
defines the VLANs that are under direct MSTP control. You cannot run 802.1D or 802.1w on any
VLAN (even outside of MSTP scope) and you cannot create topology groups when a system is under
MSTP mode. While a VLAN group will still be supported when a system is under MSTP mode, the
member VLAN should either be all in the MSTP scope or all out of the MSTP scope.
When a system is configured from non-MSTP mode to MSTP mode, the following changes are made
to the system configuration:
• All 802.1D and 802.1w STP instances are deleted regardless of whether the VLAN is inside the
MSTP scope or not
• All topology groups are deleted
• Any GVRP configuration is deleted
• Any VSRP configuration is deleted
• Single-span (if configured) is deleted
• MRP running on a VLAN inside MSTP scope is deleted
• The CIST is created and all VLANS inside the MSTP scope are attached with the CIST
Make sure that no physical layer-2 loops exist prior to switching from non-MSTP mode to MSTP mode.
If, for example, you have an Layer 2 loop topology configured as a redundancy mechanism before you
perform the switch, a Layer 2 storm should be expected.
To configure a system into MSTP mode, use the following command at the Global Configuration level.
device(config)#mstp scope all
Configuration notes
332 FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide
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