Loop Protection - NETGEAR M4200 Software Administration Manual

Loop Protection

Loops inside a network are costly because they consume resources and reduce the
performance of the network. Detecting loops manually can be cumbersome.
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 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.
You can configure the interval between two successive loop protection PDU packets with the
transmit-interval command. The default interval parameter 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 the port level, regardless of VLAN assignment and membership,
detecting loops across VLANs. The protection uses the OUI-Extended Ethernet Type frame
format (0x88b7), which is an IEEE-sanctioned format for vendor-specific packets.
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 blocking state,
loop protection packets are received and loop protection brings down the port that is involved
in the loop (if the configured action is to shut down the port).
The following example shows how you can enable loop protection on ports 1/0/1 and 1/0/2:
1. Enable loop protection globally.
(Netgear Switch) (Config) #keepalive
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