Advanced Traffic Management Guide K/KA/KB.15.15

includes all VLANs in the network. (An STP or RSTP network operates as a single-instance network.)
A region can include two types of STP instances:
1. Internal Spanning-Tree Instance (IST Instance): This is the default spanning tree instance in any
MST region. It provides the root switch for the region and comprises all VLANs configured on
the switches in the region that are not specifically assigned to Multiple Spanning Tree Instances
(MSTIs, described below). All VLANs in the IST instance of a region are part of the same,
single spanning tree topology, which allows only one forwarding path between any two nodes
belonging to any of the VLANs included in the IST instance. All switches in the region must
belong to the set of VLANs that comprise the IST instance.
2. MSTI (Multiple Spanning Tree Instance): This type of configurable spanning tree instance
comprises all static VLANs you specifically assign to it, and must include at least one VLAN.
The VLAN(s) you assign to an MSTI must initially exist in the IST instance of the same MST
region. When you assign a static VLAN to an MSTI, the switch removes the VLAN from the
IST instance. (Thus, you can assign a VLAN to only one MSTI in a given region.) All VLANs
in an MSTI operate as part of the same single spanning tree topology. (The switch does not
allow dynamic VLANs in an MSTI.)
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 paper, an MSTP bridge is an 8200zl switch (or another 802.1s-compatible
device) configured for MSTP operation.
MST Region: An MST region comprises the VLANs configured on physically connected MSTP
switches. All switches in a given region must be configured with the same VLANs and Multiple
Spanning Tree Instances (MSTIs). The 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:
1. All switches have identical MST configuration identifiers (region name and revision number).
2. All switches have identical VLAN assignments to the region’s IST and (optional) MST instances.
3. One switch functions as the designated bridge (IST root) for the region.
4. No switch has a point-to-point connection to a bridging device that cannot process RSTP
BPDUs.
CAUTION: When you enable MSTP on the switch, the default MSTP spanning tree configuration
settings comply with the values recommended in the IEEE 802.1s MSTP standard. Note that
inappropriate changes to these settings can result in severely degraded network performance. For
this reason, HP strongly recommends that changing these default settings be reserved only for
experienced network administrators who have a strong understanding of the IEEE 802.1D/w/s
2004 standards and operation.
452 Spanning tree interoperability between HP and Cisco switches