Dell N3200-ON Networking N-Series Switches User Guide version 6.8.0 - Page 905

What is Loop Protection? Dell Networking implements a subset of the Configuration Testing Protocol (CTP) for the detection of network loops. The Configuration Testing Protocol is part of the original Ethernet specification. It does not appear in the IEEE 802 standard. The Dell implementation of the Loop Protocol unicasts a CTP reply packet with the following field settings: Source MAC Address: switch L3 MAC address Destination MAC Address: switch L3 MAC address Ether Type: Skip Count: 0x9000 (LOOP) 0 Functions: Reply Receipt Number: 0 Data: 0 If any interface receives CTP packets with the switch's MAC address as the source and the number of such packets received is in excess of the configured limit, the interface is error-disabled with a Loop Protection cause. The default limit is three packets received. Since all switch ports share the same MAC address, multiple ports may be disabled by a network loop. Disabled ports may be configured to be brought back into service by the Error Recovery feature. The switch never sends a response to received CTP packets. The switch may flood the first few CTP packets it receives until a MAC address entry is placed in the CAM. The CTP protocol operates on physical Ethernet interfaces only. It does not operate over Link Aggregation Groups. The CTP protocol does not operate over the out-of-band interface. The CTP protocol is disabled on all physical Ethernet interfaces by default. CTP packet reception is not blocked by spanning tree. CTP should be enabled only on interfaces that are not running spanning-tree as it may disable spanning-tree designated (blocked) ports. Port-Based Traffic Control 905