D-Link DGS-3630 Emulator 1 - Page 1504

DGS-3630 Layer 3 Stackable Managed Switch CLI Reference Guide The STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. In its operation, STP relies on continuous reception or transmission of BPDUs based on the port role. The designated port transmits BPDUs, and the non-designated port receives BPDUs. When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes designated and moves to a forwarding state. This situation creates a loop. Example This example shows how to enable the loop guard mode on port 3. Switch#configure terminal Switch(config)#interface eth1/0/3 Switch(config-if)#spanning-tree loop-guard Switch(config-if)# 117-18 spanning-tree guard root This command is used to enable the root guard mode. Use the no form of this command to revert to the default setting. spanning-tree guard root no spanning-tree guard root Parameters None. Default By default, this option is disabled. Command Mode Interface Configuration Mode. Command Default Level Level: 12. Usage Guideline BPDU guard prevents a port from becoming a root port. This feature is useful for the service provider to prevent external bridges to a core region of the network influencing the spanning tree active topology, possibly because those bridges are not under the full control of the administrator. When a port is guarded from becoming a root port, the port will only play the role as a designated port. If the port receives the configuration BPDU with a higher priority, the port will change to the alternate port, which is in the blocking state. The received superior factor will not participate in the STP computation. The port will listen for BPDUs on the link. If the port times out the received superior BPDU, it will change to the designated port role. When a port changes to the alternate port state, due to the root guard, a system message will be generated. This configuration will take effect for all the spanning-tree versions. 1499