Multiple Spanning Tree Protocol Overview; Mstp Regions - Cisco ASR 9000 Series Configuration Manuallines

Multiple Spanning Tree Protocol Overview

Multiple Spanning Tree Protocol Overview
The Multiple Spanning Tree Protocol (MSTP) is an STP variant that allows multiple and independent spanning
trees to be created over the same physical network. The parameters for each spanning tree can be configured
separately, so as to cause a different network devices to be selected as the root bridge or different paths to be
selected to form the loop-free topology. Consequently, a given physical interface can be blocked for some of
the spanning trees and unblocked for others.
Having set up multiple spanning tree instances, the set of VLANs in use can be partitioned among them; for
example, VLANs 1 - 100 can be assigned to spanning tree instance 1, VLANs 101 - 200 can be assigned to
spanning tree instance 2, VLANs 201 - 300 can be assigned to spanning tree instance 3, and so on. Since each
spanning tree has a different active topology with different active links, this has the effect of dividing the data
traffic among the available redundant links based on the VLAN—a form of load balancing.

MSTP Regions

Along with supporting multiple spanning trees, MSTP also introduces the concept of regions. A region is a
group of devices under the same administrative control and have similar configuration. In particular, the
configuration for the region name, revision, and the mapping of VLANs to spanning tree instances must be
identical on all the network devices in the region. A digest of this information is included in the BPDUs sent
by each device, so as to allow other devices to verify whether they are in the same region.
The following figure shows the operation of MST regions when bridges running MSTP are connected to
bridges running legacy STP or RSTP. In this example, switches SW1, SW2, SW3, SW4 support MSTP, while
switches SW5 and SW6 do not.
Figure 54: MST Interaction with Non-MST Regions
To handle this situation, an Internal Spanning Tree (IST) is used. This is always spanning tree instance 0
(zero). When communicating with non-MSTP-aware devices, the entire MSTP region is represented as a
single switch. The logical IST topology in this case is shown in the following figure.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
414
Implementing Multiple Spanning Tree Protocol