Filter
Top Products
Multiple Spanning Tree Protocol | 643
29
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 29-1, 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 29-1 demonstrates
how you can use MSTP to achieve load balancing.
Figure 29-1. 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