Spanning Tree Features; Loop Protect - Enterasys Matrix DFE-Gold Series Configuration Manual

Overview: Single, Rapid and Multiple Spanning Tree Protocols
For details on creating Spanning Tree instances, refer to "set spantree msti" on page 6‐14.
For details on mapping Spanning Tree instances to VLANs, refer to "set spantree mstmap" on
page 6‐15.

Spanning Tree Features

The Matrix Series 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
listening if it fails to get a response. This protects against misconfiguration and protocol failure by
the connected bridge.
The Disputed BPDU mechanism protects against looping in situations where there is one way
communication. A disputed BPDU is one in which the flags field indicates a designated role and
learning and the priority vector is worse than that already held by the port. If a disputed BPDU is
received, the port is forced to the listening state. When an inferior designated BPDU with the
learning bit set is received on a designated port, its state is set to discarding to prevent loop
formation. Note that the Dispute mechanism is always active regardless of the configuration
setting of Loop Protection.
Loop Protect operates as a per port, per MST instance feature. It should be set on inter‐switch
links. It is comprised of several related functions:
• Control of port forwarding state based on reception of agreement BPDUs
• Control of port forwarding state based on reception of disputed BPDUs
• Communicating port non‐forwarding status through traps and syslog messages
6-2 Spanning Tree Configuration
Note: MSTP and RSTP are fully compatible and interoperable with each other and with legacy STP
802.1D.