Users Guide

838 Spanning Tree Protocol
Enabling loop guard prevents such accidental loops. When a port is no longer
receiving BPDUs and the max age timer expires, the port is moved to a loop-
inconsistent blocking state. In the loop-inconsistent blocking state, traffic is
not forwarded so the port behaves as if it is in the blocking state; that is, it
discards received traffic, does not learn MAC addresses, and is not part of the
active topology. The port will remain in this state until it receives a BPDU. It
will then transition through the normal spanning tree states based on the
information in the received BPDU.
BPDU Protection
When the switch is used as an access-layer device, most ports function as
edge ports that connect to a device such as a desktop computer or file server.
The port has a single, direct connection and is configured as an edge port to
implement the fast transition to a forwarding state. When the port receives a
BPDU packet, the system sets it to non-edge port and recalculates the
spanning tree, which causes network topology flapping. In normal cases, these
ports do not receive any BPDU packets. However, someone may forge BPDU
to maliciously attack the switch and cause network flapping.
BPDU protection can be enabled in RSTP to prevent such attacks. When
BPDU protection is enabled, the switch disables an access port that has
received a BPDU and notifies the network manager about it.
RSTP-PV
Dell EMC Networking N-Series switches support both Rapid Spanning Tree
Per VLAN (RSTP-PV) and Spanning Tree Per VLAN (STP-PV) with a high
degree of interoperability with other vendor implementations, such as Cisco's
PVST+ and RPVST+. RSTP-PV is the IEEE 802.1w (RSTP) standard
implemented per VLAN. A single instance of rapid spanning tree (RSTP)
runs on each configured VLAN. Each RSTP instance on a VLAN has a root
switch. The RSTP-PV protocol state machine, port roles, port states, and
timers are similar to those defined for RSTP. RSTP-PV embeds the DRC and
IndirectLink Fast Rapid Convergence (IRC) features, which cannot be
disabled.
NOTE: Loop Guard should be configured only on non-designated ports. These
include ports in alternate or backup roles. Root ports and designated ports
should not have loop guard enabled so that they can forward traffic.