Loop detection on access switch before core switch shuts down the uplinks

[mohammad_shamse]

Dears,

I'm running into a problem where i have a setup of several access cabinets connected via a star topology to core switch, access switches are icx 7150 and 7250 and core is 2 stacked 7650. We configured loose mode loop protection on all of the access cabinets as well as on the access switches. MSTP also configured, all vlans are in one instance where core is the root bridge. The problem happening is that the core switch is disabling one uplink configured as a lag to one of the edge cabinets, when viewing the logs on the core , i found the below : LOOP DETECTION: VLAN 102, port lg110 detect, putting into err-disable state

after disabling, reenabling the lag ports on the core, the uplink was up again, i viewed the logs on the edge cabinets and the access ports tx rx statistics, but there was no clue about a loop. The problem happened again after two days and i'm not able to locate the loop source. So please any advise about how i would locate the source of the issue, moreover, why the core is shutting down the port before the edge switch shuts down the access port which causing the issue ? since loop protect is configured on both ... 

Thanks for your help and Regards 

[SHAMZ · Accepted solution]

Hello Shams,

You can configure the loop detection in two ways, loose mode and strict mode. The strict mode is based on interface and loose mode based on VLAN. 

The action scenario is that, the strict mode will disable the port if a packet is looped back to that same port. 

In Loose Mode, loop detection is configured on the VLAN basis. Loose Mode disables the port if packets originate from any port or VLAN on the same device. 

So please configure the loop detection in loose mode at IDF side, your problem will be solved.

Please give kudos, if your issue solved.

[BenBeck]

Hey @mohammad_shamse,

Shams described the types of loop detection correctly. Loose mode can be a bit tricky since it will disable ports if a control packet is looped back anywhere on the same vlan. You can force loop detection to *not* disable certain ports if they are particularly important:

https://docs.commscope.com/bundle/fastiron-08090-managementguide/page/GUID-A45730FC-CC71-43C5-B21F-A...

Alternatively, you can also have the ports auto-enable:

https://docs.commscope.com/bundle/fastiron-08090-managementguide/page/GUID-7FB55211-41A6-46FC-A697-4...

If you need help troubleshooting a loop, I would advise you to open a support case (see my signature). 

[stefan_ilic]

Hi all,

I need help, and I think the problem is very similar.

I have a network in star, it started to happen that MSTP turns off the link ports to the Core switch (2x 7450 in stack), and then at some point all the links on the Core switch drop.

MSPT scoop is set on all switches, all VLANs have instance 0.

No spanning tree is set in all VLANs, and spanning tree root-protect is set on ports.

Do you have any suggestions on what I could do to help me solve this problem?

Best Regards.