Broadcasting VLAN 1

[Gumby]

Hi all,

Updated my switches from 9.32 to 9.34 yesterday.  Having a nice surprise this morning on my stack.  Seems like everything on VLAN 1 is being broadcast to all switch ports.

Not affecting other switches that are trunked off the stack, just everything within the stack, and only on VLAN 1.  Only 2 switches in the stack, a 225-24P and 225-48LP.

Anyone had something like this before?  Unfortunately can't do a reboot of the stack until this evening.

[PhilipDAth]

Haven't had that one! Oh joy.

You sure it isn't traffic to a specific MAC address (and broadcast and multicast traffic) that is being broadcast to every port?

Packets will be "broadcast" if the destination is not in the mac address table (aka, a unknown unicast flood).

[Gumby]

Had a look at a packet capture from a port in wireshark.  Getting point to point TCP traffic being delivered to ports that shouldn't be receiving it.

It just seems to be behaving as a hub for some reason.  Issues at boot on the stack and it has decided to broadcast as a fallback mechanism?

[PhilipDAth]

The destination IP address of a packet that is appearing on a wrong port - if you go to that machine and ping something does the issue stop?

This would still be an unknown unicast flood.  The switch can only flood packets until it knows where the destination is located.

[Gumby]

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@PhilipDAth wrote:

The destination IP address of a packet that is appearing on a wrong port - if you go to that machine and ping something does the issue stop?

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Nope, issue continues.

Had some time to do some more testing.  Transferring from one host to another on the same physical switch is fine.  As soon as you transfer something across switches in the stack it seems to broadcast.

[Gumby]

Just to clarify this a little more, I think....

                      Stack

Switch 4 -- Trunk -- Switch 1

                     Switch 2 -- Trunk -- Switch 3

Within switch 1 is okay

Within switch 2 is okay

Switch 2 to switch 3 is okay

Switch 4 to Switch 3 is okay traversing the stack.

Switch 1 to Switch 2 is broadcasting on both switch 1 and 2

Switch 1 to Switch 3 is broadcasting on switch 1 and 2 but not 3.

Haven't heard back from Meraki support at all today.  Just going to give the stack a restart tonight and see if it clears the issue.

[PhilipDAth]

I'm going to assume you have not turned spanning tree off (it is on by default), and all the switches are Cisco Meraki switches, so we have a loop free environment.

It sounds like the stack has failed to correctly replicate the mac forwarding table.  This sounds like a software bug.  I agree, reboot the stack.

[Gumby]

Rebooted the second switch, no change.  Rebooted the first switch, back to normal!

Definitely something a bit screwed up in the replication.  Easy fix in the end, just a pity support didn't take a look at it while in that state to see what the real problem was.

[Gumby · Accepted solution]

Heard back from Meraki support early this morning:

"It seems this is a known issue, albeit very rare. I have attached this case to the bug with our dev team, however, for the time being please reboot the switches. In similar cases the issue seems to go away after doing so."