Administrator Guide

Port-Based Traffic Control 853
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:
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 enabled on all physical Ethernet interfaces by default.
CTP packet reception may blocked by spanning tree. With the default
settings, spanning tree will detect a loop in the network prior to CTP and set
the affected interfaces into the spanning tree blocked state.
Source MAC Address: switch L2 MAC address
Destination MAC Address: switch L2 MAC address
Ether Type: 0x9000 (LOOP)
Skip Count: 0
Functions: Reply
Receipt Number: 0
Data: 0