Users Guide

Port-Based Traffic Control 943
What is Loop Protection?
Dell EMC 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 EMC implementation of the Loop Protocol unicasts a CTP reply
packet with the following field settings:
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.
Source MAC Address: switch L3 MAC address
Destination MAC Address: switch L3 MAC address
Ether Type: 0x9000 (LOOP)
Skip Count: 0
Functions: Reply
Receipt Number: 0
Data: 0