Multiple Spanning Tree Protocol (Mstp); Protocol Overview - Dell Force10 Z9000 Configuration Manual

Multiple Spanning Tree Protocol (MSTP)

Multiple Spanning Tree Protocol (MSTP)

Protocol Overview

Multiple Spanning Tree Protocol (MSTP)—specified in IEEE 802.1Q-2003—is an RSTP-based spanning
tree variation that improves on PVST+. MSTP allows multiple spanning tree instances and allows you to
map many VLANs to one spanning tree instance to reduce the total number of required instances.
In contrast, PVST+ allows a spanning tree instance for each VLAN. This 1:1 approach is not suitable if
you have many VLANs, because each spanning tree instance costs bandwidth and processing resources.
In the following illustration, three VLANs are mapped to two Multiple Spanning Tree instances (MSTI).
VLAN 100 traffic takes a different path than VLAN 200 and 300 traffic. The behavior demonstrates how
you can use MSTP to achieve load balancing.
Figure 27-1. MSTP with Three VLANs Mapped to Two Spanning Tree Instances
MSTI 2 root
is supported on platforms:
R1 MSTI 1: VLAN 100
MSTI 2: MSTI 2: VLAN 200, VLAN 300 VLAN
1/21
1/31
3/11
3/21
R3
e c s z
R2
200, VLAN 300
2/11
2/31
Multiple Spanning Tree Protocol (MSTP) | 591
27
MSTI 1 root