Product guide

Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
Common Spanning Tree (CST): Refers to the single forwarding path the
switch calculates for STP (802.1D) and RSTP (802.1w) topologies, and for
inter-regional paths in MSTP (802.1s) topologies. Note that all three types of
spanning tree can interoperate in the same network. Also, the MSTP switch
interprets a device running 802.1D STP or 802.1w RSTP as a separate region.
(Refer to figure 6-25 on page 6-47.)
Internal Spanning Tree (IST): Comprises all VLANs within a region that
are not assigned to a multiple spanning-tree instance configured within the
region. All MST switches in a region should belong to the IST. In a given region
“X”, the IST root switch is the regional root switch and provides information
on region “X” to other regions.
MSTP (Multiple Spanning Tree Protocol): A network supporting MSTP
allows multiple spanning tree instances within configured regions, and a
single spanning tree among regions, STP bridges, and RSTP bridges.
MSTP BPDU (MSTP Bridge Protocol Data Unit): These BPDUs carry
region-specific information, such as the region identifier (region name and
revision number). If a switch receives an MSTP BPDU with a region identifier
that differs from its own, then the port on which that BPDU was received is
on the boundary of the region in which the switch resides.
MSTP Bridge: In this manual, an MSTP bridge is a 5300xl, 4200vl, 3400cl, or
6400cl switch (or another 802.1s-compatible device) configured for MSTP
operation.
MST Region: An MST region forms a multiple spanning tree domain and is a
component of a single spanning-tree domain within a network. For switches
internal to the MST region:
■ All switches have identical MST configuration identifiers (region name
and revision number).
■ All switches have identical VLAN assignments to the region’s IST and
(optional) MST instances.
■ One switch functions as the designated bridge (IST root) for the region.
■ No switch has a point-to-point connection to a bridging device that cannot
process RSTP BPDUs.
Operating Rules
■ All switches in a region must be configured with the same set of VLANs,
as well as the same MST configuration name and MST configuration
number.
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