Core issue
A master loop breaker election causes the %GIGASTACK-6-LOOP_DETECTED: Gigastack GBIC in interface name is selected as Master Loop Breaker message to be displayed on the elected switch.
Resolution
In a redundant loop configuration, an election of which GigaStack link is to be blocked occurs. This is also known as master loop breaker. All the LEDs on all GigaStacks flash amber. Once the master loop breaker has been elected, all GigaStack LEDs are solid green, with the exception of the blocked master loop breaker. That link is solid amber.
A message similar to this is displayed on the elected master loop breaker switch:
%GIGASTACK-6-LOOP_DETECTED: Gigastack GBIC in Gi0/1 is selected as Master Loop Breaker.
Link 2 of the master loop breaker GigaStack Gigabit Interface Converter (GBIC) is disabled to break the loop. Issuing the show interface command on the master loop breaker port displays this message:
GigaStack module(0.1) in GBIC slot. link1 is up, link2 is redundant link.
This is normal operation and the message is for informational purposesonly. During this time, you may see the links go up or down multiple times. This is normal behavior.
The loop breaking algorithm occurs between the GigaStack GBICs by exchanging MAC addresses using their A- and B-ports. The winner of the election becomes the master loop breaker and disable its port 2. The winner of the election is the one with the lowest byte-order reversed MAC address. For example, if a gig port has the MAC address 00:d0:79:6b:ee:01, the byte-order reversed MAC address is 01:EE:6b:79:d0:00.
To find out which switch disables its B-port, issue the show interfaces gigabitethernet (sh int gig) command on each switch, and make note of the MAC address in parentheses (the one that indicates bia followed by a MAC address). With this information, perform the byte-order reversing operation, and compare them to find the smallest one. The smallest one is the master loop breaker and disables its port 2.
For more information about GigaStack configuration, refer to Catalyst Switch GigaStack Configuration and Implications.
