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290 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
802.1s Multiple Spanning Tree Protocol
8
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 L2 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.
PowerConnect(config)#mstp scope all
Syntax: [no] mstp scope all
NOTE
MSTP is not operational however until the mstp start command is issued as described in “Activating
MSTP on a switch” on page 297.
Once the system is configured into MSTP mode, CIST (sometimes referred to as “instance 0”) is
created and all existing VLANs inside the MSTP scope are controlled by CIST. In addition, whenever
you create a new VLAN inside MSTP scope, it is put under CIST control by default. In the Dell MSTP
implementation however, a VLAN ID can be pre-mapped to another MSTI as described in
“Configuring an MSTP instance” on page 294. A VLAN whose ID is pre-mapped, will attach to the
specified MSTI instead of to the CIST when created.