Spanning tree interoperability between HP and Cisco switches > Terminology

  

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Terminology

Common and Internal Spanning Tree (CIST): Comprises all LANs, STP, and RSTP bridges and MSTP regions in a network. The CIST automatically determines the MST regions in a network and defines the root bridge (switch) 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): Administers the connectivity among the MST regions, STP LANs, and RSTP LANs in a bridged network. 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 versions 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.

Internal Spanning Tree (IST): When you configure a switch for MSTP operation, the switch automatically includes all of the static VLANs configured on the switch in a single, active spanning tree topology (instance) within the IST. This is termed the “IST instance”. Any VLANs you subsequently configure on the switch are added to this IST instance. To create separate forwarding paths within a region, group specific VLANs into different Multiple Spanning Tree Instances (MSTIs).

Multiple Spanning Tree Instances: A multiple spanning tree network comprises separate spanning-tree instances existing in an MST region. (There can be multiple regions in a network.) Each instance defines a single forwarding topology for an exclusive set of VLANs. By contrast, an STP or RSTP network has only one spanning tree instance for the entire network, and

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: ]

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.

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