Enterasys Networks 9034313-07 manual Spanning Tree Features, Loop Protect

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.

Note: MSTP and RSTP are fully compatible and interoperable with each other and with legacy STP 802.1D.

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.

Note: The term “bridge” is used as an equivalent to the term “switch” or “device” in this document.

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