Multiple Spanning Tree; Configuring A Bridge - Motorola WiNG 4.4 Reference Manual

4 - 152 WiNG 4.4 Switch System Reference Guide

4.10 Multiple Spanning Tree

Multiple Spanning Tree Protocol (MSTP) provides a VLAN-aware protocol and algorithm to create and maintain a loop-free
network. It allows the configuration of multiple spanning tree instances. This ensures a loop-free topology for one or more
VLANs. It allows the network administrator to provide a different path for each group of VLANs to better utilize redundancy.
MSTP allows rapid convergence similar to Rapid Spanning Tree Protocol (RSTP). RSPT is an independent protocol and
MSTP does not use RSTP. Since MSTP allows VLANs to be grouped in an instance, each instance can have its own
spanning-tree topology independent of other spanning-tree instances. This architecture provides multiple forwarding links
for data traffic and load balancing, and therefore reduces the number of spanning-tree instances required to support a
large number of VLANs.
Using MSTP, the network can be divided into regions. All switches within a region use the same VLAN to instance mapping.
The entire network runs a spanning tree instance called the Common Spanning Tree instance (CST) that interconnects
regions as well as legacy (STP and RSTP) bridges. The regions run on a local instance for each configured MSTP instance.
The network-wide spanning tree for instance 0 is known as the Common Spanning Tree (CST). A spanning tree for any other
instance, which is local to a region, is known as an Internal Spanning Tree (IST). The Common and Internal Spanning Tree
(CIST) (which consists of the CST as well as all ISTs across regions) interconnects all bridges in the network.
The following definitions describe the STP instances that define an MSTP configuration:
• Common Spanning Tree (CST) – MSTP runs a single spanning tree instance (called the Common Spanning Tree) that
interconnects all the bridges in a network. This instance treats each region as a single bridge. In all other ways, it
operates exactly like Rapid Spanning Tree (RSTP).
• Common and Internal Spanning Trees (CIST) – CIST contains all of the ISTs and bridges not formally configured into a
region. This instance inter-operates with bridges running legacy STP and RSTP implementations.
• Multiple Spanning Tree Instance (MSTI) – The MSTI is identified by an MSTP identifier (MSTPid) value from 1 to 15.
This defines an individual instance of a spanning tree. One or more VLANs can be assigned to an MSTI. A VLAN cannot
be assigned to multiple MSTIs. The multiple spanning tree instance 0 is always present. VLANs not explicitly assigned
to an instance are assigned to instance 0.
• MSTP Region – These are clusters of bridges that run multiple instances of the MSTP protocol. Multiple bridges detect
they are in the same region by exchanging their configuration digest (which is dependent on the VLAN to instance
mapping), region name, and revision-level. If you need to have two bridges in the same region, the two bridges must
have identical VLAN to instance mappings, region names, and revision-levels.
To configure the switch for MSTP support, configure the region name and the revision on each switch being configured.
This region name is unique to each region. Then create one or more instances and assign IDs. VLANs are then assigned to
instances. These instances must be configured on switches that interoperate with the same VLAN assignments. Port cost,
priority, and global parameters can then be configured for individual ports and instances.
The Multiple Spanning Tree option contains separate tabs for the following activities:
•

Configuring a Bridge

• Viewing and Configuring Bridge Instance Details
• Configuring a Port
• Viewing and Configuring Port Instance Details
4.10.1 Configuring a Bridge
Use the Bridge tab to configure the Bridge. This window displays bridge configuration details for the switch