Configuration Guide User guide

FastIron Configuration Guide 1171
53-1002494-02
802.1s Multiple Spanning Tree Protocol
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.
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