Switches - Dell PowerConnect J-EX4200-24T Software Manual

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Understanding Loop Protection for STP, RSTP, VSTP, and MSTP on J-EX Series

Switches

Disabling the BPDU protection configuration does not unblock the interface.
Example: Configuring BPDU Protection on STP Interfaces to Prevent STP
Miscalculations on J-EX Series Switches on page 1317
Example: Configuring BPDU Protection on non-STP Interfaces to Prevent STP
Miscalculations on J-EX Series Switches on page 1321
Understanding Loop Protection for STP, RSTP, VSTP, and MSTP on J-EX Series
Switches on page 1279
Understanding Root Protection for STP, RSTP, VSTP, and MSTP on J-EX Series Switches
on page 1280
Understanding MSTP for J-EX Series Switches on page 1277
Understanding RSTP for J-EX Series Switches on page 1276
Understanding STP for J-EX Series Switches on page 1275
Understanding VSTP for J-EX Series Switches on page 1281
J-EX Series Switches provide Layer 2 loop prevention through Spanning Tree Protocol
(STP), Rapid Spanning Tree Protocol (RSTP), VLAN Spanning Tree Protocol (VSTP),
and Multiple Spanning Tree Protocol (MSTP). Loop protection increases the efficiency
of STP, RSTP, and MSTP by preventing ports from moving into a forwarding state that
would result in a loop opening up in the network.
A loop-free network in spanning-tree topologies is supported through the exchange of
a special type of frame called bridge protocol data unit (BPDU). Peer STP applications
running on the switch interfaces use BPDUs to communicate. Ultimately, the exchange
of BPDUs determines which interfaces block traffic (preventing loops) and which
interfaces become root ports and forward traffic.
However, a blocking interface can transition to the forwarding state in error if the interface
stops receiving BPDUs from its designated port on the segment. Such a transition error
can occur when there is a hardware error on the switch or software configuration error
between the switch and its neighbor.
When loop protection is enabled, the spanning-tree topology detects root ports and
blocked ports and makes sure both keep receiving BPDUs. If a loop-protection-enabled
interface stops receiving BPDUs from its designated port, it reacts as it would react to a
problem with the physical connection on this interface. It doesn't transition the interface
to a forwarding state, but instead transitions it to a loop-inconsistent state. The interface
recovers and then it transitions back to the spanning-tree blocking state as soon as it
receives a BPDU.
We recommend that you enable loop protection on all switch interfaces that have a
chance of becoming root or designated ports. Loop protection is most effective when
Chapter 64: Spanning-Tree Protocols—Overview
1279