The STP and RSTP protocols allow only one spanning tree instance, instance 0, which the BIG-IP® system creates automatically when you enable spanning tree. When
running STP or RSTP, you can modify the properties of instance 0, but you cannot create
When you are running MSTP, however, the MSTP algorithm can explicitly create instances. The reason that you can create instances is that MSTP recognizes VLANs. By creating an instance and assigning one or more VLANs to it, you can control bridge loops and redundant paths within those VLANs.
For example, suppose you have two interfaces. One interface is assigned to VLAN A, while the
other interface is assigned to VLANs A and B. If you are using the STP or RSTP protocol, both of
which disregard VLANs, the protocol might block traffic for both VLANs, as shown in this
By contrast, the MSTP protocol can make blocking decisions on a per-VLAN basis. In our example,
on the interface that carries traffic for two VLANs, you can block traffic for VLAN A, but leave
a path open for VLAN B traffic. For example:
Because all BPDUs exchanged within a region always reference instance 0, instance 0 is active on all interfaces. This, in turn, can cause blocking problems. To avoid this, make sure that each VLAN on a BIG-IP system is a member of an instance that you explicitly create, rather than a member of instance 0 only. For example, suppose you create the following:
Instance 1 with VLAN A as a member, where VLAN A is associated with interface 1.2
Instance 2 with VLAN B as a member, where VLAN B is associated with interface 1.4
In this case, neither interface will be blocked, because the BPDUs sent from each interface reference a unique instance (either instance 1 or instance 2).
Because all BPDUs exchanged within a region always reference instance 0, thereby causing instance 0 to be active on all interfaces, unwanted blocking problems can occur. To avoid this, make sure that each VLAN on a BIG-IP system is a member of an instance that you explicitly create, rather than a member of instance 0 only.