and designated port for each region. The CIST includes the Common Spanning
Tree (CST), the Internal Spanning Tree (IST) within each region, and any
multiple spanning-tree instances (MSTIs) in a region.
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 6-45.)
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, 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.
Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
6-51