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New Member

multicast causes mac address flapping in vlan

I’m having a problem with multicast causing flapping between a trunk port and an access port.

Switch A is the core (a 4507)

Switch B (a 3850) is trunked to the core with multiple vlans. When I connect a sender and a receiver to switch B, after a little bit, and seemingly at random, I will get a message on the console of switch B saying some mac address on vlan 50 is flapping between the port configured as the trunk and the multicast sender.

On the 4507, vlan 50 has an ip address and is set to pim dense mode

On the 3850, there is also an ip address but no pim

Both switches have the same vlans on the trunk.

There is no physical loop and I can’t figure out what is causing the flapping.

Does anyone have any ideas?

Thanks

  • LAN Switching and Routing
6 REPLIES

multicast causes mac address flapping in vlan

Hello

Can you post the console log messages relating to this issue?

res

Paul

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Thanks.

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New Member

Re: multicast causes mac address flapping in vlan

Jan 10 16:38:48.303: %SW_MATM-4-MACFLAP_NOTIF: Host 0026.b9b1.0fc2 in vlan 50 is flapping between port Gi1/0/48 and port Gi2/0/21

Jan 10 16:38:48.397: %SW_MATM-4-MACFLAP_NOTIF: Host 0026.b9b1.0fc2 in vlan 50 is flapping between port Gi1/0/48 and port Gi2/0/21 (RDIS-FCS-01-2)

port port gi1/0/48 has this config (and same on other side of the trunk)

switchport trunk allowed vlan 50,200-202,204-214,216,250,300-352

switchport mode trunk

nmsp attachment suppress

config on ports 19 (receiver) and 21 (sender) looks as follows:

switchport access vlan 50

switchport mode access

nmsp attachment suppress

spanning-tree portfast

Re: multicast causes mac address flapping in vlan

Hello

I dont know what this command is doing? - nmsp attachment suppress,  once this command is enabled  does the loop begin after a short while?

The above log is as you know relating to a loop -  Loops occur mostly because of a port that should be in a blocking state is now forwarding traffic for some reason.

Have you checked the switches for vlan 50 -  this should give you a good idea what port is the root cause?

Sh int trunk

Sh span vlan 50

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

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New Member

multicast causes mac address flapping in vlan

nsmp attachment suppress is needed on the 3850 to disable ip tracking (which is on by default and cannot be disabled with the no form. There was a post on these forums about it.)

Below is the output of the two commands on Switch B (the 3850). As far as I know, the root at the "top" should be Switch A at the other end of my trunk

#sh interfaces trunk

Port        Mode             Encapsulation  Status        Native vlan
Gi1/0/48    on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi1/0/48    50,200-202,204-214,216,250,300-352

Port        Vlans allowed and active in management domain
Gi1/0/48    50,200-202,204-214,216,250,300-352

Port        Vlans in spanning tree forwarding state and not pruned
Gi1/0/48    50,200-202,204-214,216,250,300-352

show span vlan 50

VLAN0050
  Spanning tree enabled protocol rstp
  Root ID    Priority    12338
             Address     6c20.5692.8c80
             Cost        4
             Port        48 (GigabitEthernet1/0/48)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32818  (priority 32768 sys-id-ext 50)
             Address     5017.ff71.8f80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/48            Root FWD 4         128.48   P2p
Gi2/0/19            Desg FWD 4         128.83   P2p Edge
Gi2/0/21            Desg FWD 4         128.85   P2p Edge

Re: multicast causes mac address flapping in vlan

Hello

Check for:
Old span sessions with ports that are now access ports

Stp bpdu-filter enabled anywhere

Then check the stp path for this vlan  50 - from the root down - map out all its fwd and non fwd-ports and you should be able to find the switch and port causing this.

And finally to aide in you discovery, disable the console mesage for this loop before you begin

conf t

logging console 1

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

Please don't forget to rate any posts that have been helpful. Thanks.
New Member

multicast causes mac address flapping in vlan

Just wanted to followup and say that while trying to investigate this, the problem sems to have gone away as mysteriously as it started. I'm thinking there was broken switch or some other device plugged in at a users desk that was causing the problem.

Thanks

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