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Multiple Spanning Tree Protocol | 659
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Multiple Spanning Tree Protocol
Multiple Spanning Tree Protocol is supported on platforms: c e s
MSTP addressing is supported on the E-Series ExaScale platform with FTOS 8.1.1.0 and later.
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 Figure 282, 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 in Figure 282 demonstrates how
you can use MSTP to achieve load balancing.
Figure 30-1. MSTP with Three VLANs Mapped to Two Spanning Tree Instances
R1
R2
R3
1/21
3/11
3/21
1/31
2/11
2/31
MSTI 1 root
MSTI 1: VLAN 100
MSTI 2: VLAN 200, VLAN 300 MSTI 2: VLAN 200, VLAN 300
MSTI 2 root
Forwarding
Blocking