NETGEAR AV Line M4250 GSM4210PX 8-Port Gigabit PoE+ Compliant Managed AV Switch with SFP (220W) - Manual (Page 333)

The switch can automatically identify loops in the network. You can enable loop
protection per port or globally.
If loop protection is enabled, the switch sends predefined protocol data unit (PDU)
packets to a Layer 2 multicast destination address (09:00:09:09:13:A6) on all ports for
which the feature is enabled. You can selectively disable PDU packet transmission for
loop protection on specific ports even while port loop protection is enabled. If the switch
receives a packet with the previously mentioned multicast destination address, the
source MAC address in the packet is compared with the MAC address of the switch. If
the MAC address does not match, the packet is forwarded to all ports that are members
of the same VLAN, just like any other multicast packet. The packet is not forwarded to
the port from which it was received.
If the source MAC address matches the MAC address of the switch, the switch can
perform one of the following actions, depending on how you configure the action:
• The port is shut down.
• A log message is generated. (If a syslog server is configured, the log message can
be sent to the syslog server.)
• The port is shut down and a log message is generated.
If loop protection is disabled, the multicast packet is silently dropped.
Loop protection is not intended for ports that serve as uplinks between spanning
tree–aware switches. Loop protection is designed for unmanaged switches that drop
spanning tree bridge protocol data units (BPDUs).
You need to enable the feature globally before you can enable it at the port level so
that the system policy filter can be installed.
Loop protection treats PDU packets transmission and spanning tree protocol in the
following ways:
• Loop protection and PDU packet transmission: Loop protection sends loop
protocol packets from all ports on which it is enabled. You can configure the interval
(1 to 5 seconds) between two successive loop protection PDU packets. The default
interval is 5 seconds. If the switch receives a loop protocol packet on a port for which
the action is set to shut down the port, the port can no longer receive and send
frames.
Loop protection operates at a port level, regardless of VLAN assignment and
membership, detecting loops across VLANs.
• Loop protection and Spanning tree protocol: Loop protection does not impact
end nodes and is not intended for ports that serve as uplinks between spanning
tree–aware switches. Loop protection can coexist with Spanning Tree Protocol (STP).
You can enable both loop protection and STP on a port because these features
function independently of each other. STP does not bring a port down when a loop
is detected but keeps the port in blocking state. Because PDUs are allowed in a
Main User Manual 333 Configure Switching Information
AV Line of Fully Managed Switches M4250 Series Main User Manual