C9500 port flapping issues

[tltl4866]

Hello all, please refer to the diagram above.

We have added another set of switch to our top level C9500. The diagram above is how we connect the switches to each other. When we inter-connect the switches (see the red line), we got a warning on the top level switch saying that our vlan is flapping. This issue only occurs when the bottom switches are inter-connected with each other.

We cannot figure out why the port is flapping. I have tried to look up MAC tables on both set of switches and I do not see them conflicting.

Any advice on how to diagnose the flapping issue?

[marce1000]

 - It's a bit unclear from the picture who the top level C9500 is (?). Also do you define correct root bridge (priority) for the vlans ?

 M.

[tltl4866]

Hi,

sorry for the vague diagram. Here is a more detailed diagram:

Essentially, when we interconnected the 4500 with each other (Te1/30), 9500-R's port 7 and 8 began to flap. This flapping caused a 1 minute interruption on ISP_01 and ISP_03. We can confirm that it was not the ISP's issue.

We have not found any MAC address conflict (or any conflicting strings for that matter) on the 4500 and 9500. Should we be looking at the router instead?

We have already added 3 sets of 4500 like the diagram above prior to this and we did not experience any flapping and causing ISP interruptions. We're honestly not sure where to look at.

[MHM Cisco World]

Mac flapping occurs 

1- for short period

2- permanent 

First one normal

Second one is need to find why it happened.

Which one is your case?

[tltl4866]

Hi,

We experienced the first case. However, the flapping did cause an interruption on our network (we lost connection with our isp after the switches are inter-connected).

I also want to add that this is our 4th set of switch added on the C9500. The previous 3 sets did not cause us any trouble and all of the configurations are almost identical, the only difference is the naming and port channel.

[MHM Cisco World]

for the STP are you config PVST ? are you config load balance with PVST in which one 9500 is Root for some VLAN and other 9500 is Root for rest of VLAN??

[MHM Cisco World]

You must config 9500 as primary and secondary root, this prevent any sw to be elect as root.

It appear that either old access sw elect as root (suboptimal design) or new access sw elect as know root for domain.

The mac learing which cause flapping due to change of interface stp status.

[tltl4866]

Hi, sorry for the late reply.

We believe you are on the right direction after doing some investigations. Based on our investigation, we found that most VLANs are not root bridges on C9500, is this the root cause of the flapping? Below is our plan and was wondering if it would solve the issue:

Plan: we are planning to config STP vlans root primary on the 9500-Ext-L and STP vlans root secondary on the 9500-Ext-R

Questions: Would our problem be solved if we configured the roots correctly as you pointed out? We are currently in a production environment, would assigning root priority affect the production? What else should we look out for when configuring?

Thank you so much for giving us the advice.

[MHM Cisco World]

 are VLAN is same in both Access-SW 4500 ?
if Yes and there is L2 trunk between Agg-SW 9500 "interconnect" why you need interconnect Access-SW?
Are you config any HSRP in 9500?

[Elliot Dierksen]

One diagram show a link between the two top level 9500's, and the other does not. Is there a link between those two? I would think that there should be one.

I have 2 situations where I have seen the 'mac address flapping message'. One is where there is a loop that isn't blocked by spanning tree. The other is when there are redundant links, but each side is preferring a different link. If you have a switch with 2 links in port 1 & 2, but the second switch has those links to 2 & 1, that could cause that. The reason is each side has a different view of which link is active, and thus the mac flap. You can alter that using "spanning-tree vlan <X> port-priority <Y>" on the ports, but I would encourage you to line up the physical ports numbers lowest to highest on each side and that should resolve the port priority issue.

The other possibility is an unblocked loop. That seems less likely, but it is a possibility.

[tltl4866]

Hello,

Yes! the C9500s are indeed inter-connected. I forgot to draw that part out.

I believe that the user MHM Cisco World is on the right track based on our investigation. We will be working with them for this issue.