Configuring BPDU filtering
The STP BPDU filter feature allows control of spanning tree participation on a per-port basis. It can be used to exclude specific ports from becoming part of spanning tree operations. A port with the BPDU filter enabled will ignore incoming BPDU packets and stay locked in the spanning tree forwarding state. All other ports will maintain their role.
Syntax:
spanning-tree [port-list | all] bpdu-filterno spanning-tree [port-list | all] bpdu-filterEnables or disables the BPDU filter feature on specified port(s). This forces a port to always stay in the forwarding state and be excluded from standard STP operation.
To have STP operations running on selected ports of the switch rather than every port of the switch at a time.
To prevent the spread of errant BPDU frames.
To eliminate the need for a topology change when a port's link status changes. For example, ports that connect to servers and workstations can be configured to remain outside of spanning tree operations.
To protect the network from denial of service attacks that use spoofing BPDUs by dropping incoming BPDU frames. For this scenario, BPDU protection offers a more secure alternative, implementing port shut-down and a detection alert when errant BPDU frames are received.
Ports configured with the BPDU filter mode remain active (learning and forward frames); however, spanning tree cannot receive or transmit BPDUs on the port. The port remains in a forwarding state, permitting all broadcast traffic. This can create a network storm if there are any loops (that is, trunks or redundant links) using these ports. If you suddenly have a high load, disconnect the link and disable the bpdu-filter (using the
no command).
Configuring BPDU filtering
To configure BPDU filtering on port a9, enter:
switch(config)# spanning-tree a9 bpdu-filter