Cannot find reference for this message seen in log on Nexus 9508

[Don Sullivan]

I was just wondering if anyone was familiar with this message we saw in the logs of our 9508 core switches. I tried to find a reference by searching via Google for this message minus the site specific information and could not come up with anything, as in no hits. I opened a TAC case and I had a question concerning the response because the messages just didn't say that. Here are the messages:

%-SLOT1-5-BCM_L2_LEARN_ENABLE_MTM_FLOW_CTRL: MAC Learning Enabled unit=0

%-SLOT1-5-BCM_L2_LEARN_ENABLE_MTM_FLOW_CTRL: MAC Learning Enabled unit=1

%-SLOT1-5-BCM_L2_LEARN_DISABLE_MTM_FLOW_CTRL: MAC Learning Disabled unit=0

%-SLOT1-5-BCM_L2_LEARN_DISABLE_MTM_FLOW_CTRL: MAC Learning Disabled unit=1

TAC indicated this refers to "error message was added to show mac address table exhaustion". This would mean the mac address table had hit 160000 entries when we don't normally go above 7000 for the whole campus of 150 switches. I am just wondering if anyone else has seen this message and can provide a reference to the description of the messages and the possible causes. We cleared the error messages by reloading the core switches.

Appreciate any feedback.

[thelostboydan]

In our environment with two VPC connected Nexus 9396 switches this message was generated when we move a device from one Vlan to another Vlan on an access switch.  what occurred was the MAC address appeared on both the Vlans so loop prevention was enacted.  This generated the same series of messages.  It continued for 300 seconds which matched the MAC table time out. 

Hope this helps.

[Peter Paluch]

Hi all,

I know this thread is very old, but since there is truly too little information about these logging messages, let me clarify their meaning somewhat - hopefully, this will be helpful.

The BCM_L2_LEARN_DISABLE_MTM_FLOW_CTRL and BCM_L2_LEARN_ENABLE_MTM_FLOW_CTRL messages relate to the intensity of learning new MAC addresses, or moving existing ones around, not to the utilization of the MAC address table.
Here is what happens: When a new MAC address, or a moved known MAC address, needs to be programmed into hardware, the Broadcom User Space Driver (BCM USD) send a message to the MAC Table Manager (MTM) to handle this programming. After MTM handles the programming, it sends back a message to BCM USD that the address has been dealt with.

BCM USD keeps track of the count of outstanding messages sent to MTM. If this count exceeds 15, BCM USD logs the BCM_L2_LEARN_DISABLE_MTM_FLOW_CTRL message and stops further learning of MAC addresses until this count falls under 9. These constants are hardcoded and cannot be tuned.
If these messages are being logged, it does not mean that the MAC address table is exhausted - rather, it indicates that the information about new and moved MAC addresses were coming in so fast that MTM was not able to process them that quickly, and at some point, there were 15 oustanding messages in the MTM's queue. Please note that each of these messages can contain 1 or more MAC addresses, so 15 messages for MTM represent 15 or more MAC addresses to be processed.

All in all, these messages point to an excessive MAC learn or move activity (or, less likely, on a congestion with MTM or in the communication between BCM USD and MTM), and should be approached accordingly.
Best regards,
Peter

[thelostboydan]

A small blob of MAC addresses were flap copied to 2 VLANs. When the switch stopped learning it upset some computers.

Thank you very much for the education. I’m a bit less lost now.