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Spanning Tree Configuration Summary
MSTP
The IEEE 802.1s Multiple Spanning Tree Protocol (MSTP) builds upon 802.1D and RSTP by
optimizing utilization of redundant links between switches in a network. When redundant links
exist between a pair of switches running single STP, one link is forwarding while the others are
blocking for all traffic flowing between the two switches. The blocking links are effectively used
only if the forwarding link goes down. MSTP assigns each VLAN present on the network to a
particular Spanning Tree instance, allowing each switch port to be in a distinct state for each such
instance: blocking for one Spanning Tree while forwarding for another. Thus, traffic associated
with one set of VLANs can traverse a particular inter‐switch link, while traffic associated with
another set of VLANs can be blocked on that link. If VLANs are assigned to Spanning Trees
wisely, no inter‐switch link will be completely idle, maximizing network utilization.
For details on creating Spanning Tree instances, refer to "set spantree msti" on page 9‐12.
For details on mapping Spanning Tree instances to VLANs, refer to "set spantree mstmap" on
page 9‐14.
Spanning Tree Features
The SecureStack C3 device meets the requirements of the Spanning Tree Protocols by performing
the following functions:
•
Creating a single Spanning Tree from any arrangement of switching or bridging elements.
•
Compensating automatically for the failure, removal, or addition of any device in an active
data path.
•
Achieving port changes in short time intervals, which establishes a stable active topology
quickly with minimal network disturbance.
•
Using a minimum amount of communications bandwidth to accomplish the operation of the
Spanning Tree Protocol.
•
Reconfiguring the active topology in a manner that is transparent to stations transmitting and
receiving data packets.
•
Managing the topology in a consistent and reproducible manner through the use of Spanning
Tree Protocol parameters.
Loop Protect
The Loop Protect feature prevents or short circuits loop formation in a network with redundant
paths by requiring ports to receive type 2 BPDUs (RSTP/MSTP) on point‐to‐point inter‐switch
links (ISLs) before their states are allowed to become forwarding. Further, if a BPDU timeout
occurs on a port, its state becomes listening until a BPDU is received.
Both upstream and downstream facing ports are protected. When a root or alternate port loses its
path to the root bridge due to a message age expiration it takes on the role of designated port. It
will not forward traffic until a BPDU is received. When a port is intended to be the designated port
in an ISL it constantly proposes and will not forward until a BPDU is received, and will revert to
9-2 Spanning Tree Configuration
Note: MSTP and RSTP are fully compatible and interoperable with each other and with legacy
STP 802.1D.
Note: The term "bridge" is used as an equivalent to the term "switch" or "device" in this document.
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