Hirschmann RS20 Reference Manual page 175

Redundancy
Parameters Meaning
Root Guard The "Root Guard" setting is only relevant for
ports with the STP role designated.
If such a port receives an STP-BPDU with
better path information on the root that what
the device knows, the device discards the
BPDU and sets the port status to
discarding, instead of assigning the port the
STP port role root.
Thus the device helps protect your network
from attacks with STP-BPDUs that try to
change the topology, and from incorrect
configurations.
If there are no STP-BPDUs with better path
information on the root, the device resets the
transmission status of the port according to the
port role.
Note: The "Root Guard" and "Loop Guard"
settings are mutually exclusive. If you activate
one setting when the other is already active,
the device switches off the other one.
TCN Guard If the "TCN Guard" setting is active (TCN:
Topology Change Notification) the port ignores
the topology change flag in the STP-BPDUs
received, which is reporting a topology
change.
Thus the device protects your network from
attacks with STP-BPDUs that try to change the
topology.
If the "TCN Guard" setting is inactive, the
device follows the protocol in reacting to the
STP-BPDUs received: it deletes its address
table and forwards the TCN information.
Note: If the received BPDU contains other
information apart from the topology change
flag that causes a topology change, the device
processes the BPDU even if the TCN guard is
activated. Example: the device receives better
path information for the root than that already
known.
Table 111:Port-related STP settings and displays, guards
RM GUI L2E
Release 8.0 05/2013
6.3 Spanning Tree
Possible values Default
setting
active (box
inactiv
selected),
e
inactive (box
empty)
active (box
inactiv
selected),
e
inactive (box
empty)
175