Administrator Guide

Layer 2 Switching Commands 580
User Guidelines
Loop protection operates by unicasting a Configuration Test Protocol (CTP)
reply packet with the following field settings:
• Source MAC Address:switch L3 MAC address
• Destination MAC Address: Switch L3 MAC address
• Ether Type: 0x0900 (LOOP)
• Skip Count: 0
• Functions: Reply
• Receipt Number: 0
• Data: 0
Since all switch ports share the same MAC address, if any interface receives
CTP packets transmitted by the switch in excess of the configured limit, that
interface is error disabled with a Loop Protection cause.
Looped CTP packets received on a routed interface are ignored and will not
error-disable the interface. This is because routed interfaces receiving a
packet addressed to the router will not unicast flood the packet to the VLAN.
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. It may be configured to operate on
LAG members.
The CTP protocol does not operate over the out-of-band interface.
Command History
Implemented in version 6.3.0.1 firmware.
Example
The following example enables loop protection on an interface:
console(config)#keepalive
console(config)#interface gi1/0/1
console(config-if-Gi1/0/1)#keepalive
This example disables loop protection on an interface: