Информационное Руководство для Cisco Cisco Nexus 5010 Switch
Introduction
This document describes how to address the "FWM-2-STM_LOOP_DETECT" error message in
the log of a Nexus 5000 switch.
the log of a Nexus 5000 switch.
What do you do when a Nexus 5000 switch displays the
"FWM-2-STM_LOOP_DETECT" message in the log?
"FWM-2-STM_LOOP_DETECT" message in the log?
This message indicates that the switch receives frames with the same source MAC address on
these two interfaces and that the swtich learns the same MAC address on these interfaces at a
very high speed. The switch detects this as a loop. The switch disables MAC address learning in
order to protect its control plane. This is implemented on all VLANs even if the loop occurred on
only one VLAN.
these two interfaces and that the swtich learns the same MAC address on these interfaces at a
very high speed. The switch detects this as a loop. The switch disables MAC address learning in
order to protect its control plane. This is implemented on all VLANs even if the loop occurred on
only one VLAN.
Possible Causes
MAC addresses move because of incorrect Spanning Tree Protocol (STP)-port state
convergence.
convergence.
●
MAC addresses move because the source of the data is physically moved across all switches
while STP states are converged and in correct states.
while STP states are converged and in correct states.
●
MAC addresses can move between interfaces if the server Network Interface Cards (NICs)
are configured for teaming/bonding, but the connected switch interfaces are not. This can be
avoided if you use Link Aggregation Control Protocol (
are configured for teaming/bonding, but the connected switch interfaces are not. This can be
avoided if you use Link Aggregation Control Protocol (
●
How is the loop actually detected?
Forwarding Manager (FWM) has a mechanism to count the number of MAC-move-backs and
weigh them based on the number of times the MAC address moves. It determines the total MAC-
move-backs count (switch-wide across all VLANs, MACs, and interfaces), declares the %FWM-2-
STM_LOOP_DETECT, and disables learning to protect FWM in loopy conditions.
weigh them based on the number of times the MAC address moves. It determines the total MAC-
move-backs count (switch-wide across all VLANs, MACs, and interfaces), declares the %FWM-2-
STM_LOOP_DETECT, and disables learning to protect FWM in loopy conditions.
Threshold Math: 28,000 MAC move-backs count in a given aging scan period of 10 seconds
switch-wide. It is declared as %FWM-2-STM_LOOP_DETECT and learning is disabled.
switch-wide. It is declared as %FWM-2-STM_LOOP_DETECT and learning is disabled.
Example Messages
The logic for MAC-move notifications should be noted. It is possible to notify MAC-moves when
the MAC-address-table notification for MAC-moves is enabled. This adds notification logs on the
console but no action is taken. A move is declared when a given MAC address has moved three
times back and forth across a given pair of ports on a VLAN within an aging scan period of 10
seconds.
the MAC-address-table notification for MAC-moves is enabled. This adds notification logs on the
console but no action is taken. A move is declared when a given MAC address has moved three
times back and forth across a given pair of ports on a VLAN within an aging scan period of 10
seconds.
Troubleshoot
You can enable MAC-move notification on the switch to find out which MAC addresses move.
Nexus-5000# conf t
Nexus-5000(config)# mac address-table notification mac-move
With Nexus 5000 switches, it is not always sufficient to enable the MAC-move notification in order