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New Member

Host flapping every 2 hours for 15 minutes

Hello,

i receive following log from cisco 3750 stack with IOS 12.2(50)SE3:

%SW_MATM-4-MACFLAP_NOTIF: Host 0019.07e4.0c00 in vlan 42 is flapping between port Po3 and port Po2

The flapping repeats every 2 hours and hold on 15 minutes.

The MAC belongs to the router SVI and all other SVIs too.

We didnt find any STP-Loop in this VLAN.

Does anybody have an idea what happens here?

If you need more Information let me know.

Thanks

Andreas

6 REPLIES
VIP Purple

Host flapping every 2 hours for 15 minutes

Hi Andreas,

There are lots of thread discussing about this, you should do a search before creating a new post.

Anyway, this is how you approach these types of flapping:

1. Is the the given MAC flapping in the log flapping only 1 time or you see it multiple times over a reasonobly short time?

  If you see it only once or once every 2-3 hours this might be not an issue worth being investigated. Sporadic one time flapping are expected in L2 broadcast domain.

If you see it often continue to step 2.

2. Identify and locate the flapping mac in vlan 42: 0019.07e4.0c00 Is the mac of a dual-homes server using some kind of load balancing algorithm (active/active) for which the same address is used from both NICs?

If yes, the message is not and issue but just an indication. Fix this type of LB (make it active/standby or make sure the server uses 2 different mac addresses, one per NIC) or if it is not possible leave it like this.

3. Is the MAC a the wireless NIC of a PC?

Make sure that the user was not moving from one AP to another (flapping is normal in this case)

4. If all previous answers are no start checking STP in vlan 42. Do you see an increasing number of TCNs  > type

show spanning-tree vlan 42 detail MULTIPLE TIMES and look for  topology changes number

VLAN000x is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 5, address xxxxxxxxxxxxxx

  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6

  We are the root of the spanning tree

  Topology change flag not set, detected flag not set

  Number of topology changes 1 last change occurred 3w0d ago   <<<<<<<

          from GigabitEthernet1/0/1          <<<<<<

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

See if you have increasing TCN's and check if they are coming from the same interface.

From this point on you keep on troubleshooting STP until you find the offending link (likely going up and down) or the switch. You also need to check if STP in vlan 42 is coherent with the actual L2 topology you have.

Regards

Please rate if it helps.

New Member

Host flapping every 2 hours for 15 minutes

Hi sandeepchoudhary,

thanks for your reply.

I've  tried to find other discussions about my problem. Your are right there  are different ones about "flapping". But i haven't found anything about  circular flapping.

I'll answer your questions:

1. i see multiple, circular flapping. Every 2 hours for about 15 Minutes. During the other 1,75 hours happens no flapping.

2. the MAC belongs to an SVI 42 of our Cisco 6500 backbone router. It also belongs to all other SVIs on that device.

3. it is not from a wireless device

4. the output of "show spanning-tree vlan 42 detail":

VLAN0042 is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 42, address 0024.f93e.6200

  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6

  Current root has priority 8234, address 0019.07e4.0c00

  Root port is 496 (Port-channel2), cost of root path is 4

  Topology change flag not set, detected flag not set

  Number of topology changes 11 last change occurred 2d08h ago

          from Port-channel2

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

On the switch are no offending links; the STP topology is working like on schedule.

I hope this helps to isolate our problem.

Regards

Andreas

VIP Purple

Host flapping every 2 hours for 15 minutes

Can you paste the config of port 3 and port2?

Regards

New Member

Host flapping every 2 hours for 15 minutes

interface Port-channel2

description cbb-sw4 po15

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 42,46,49

switchport mode trunk

logging event trunk-status

logging event bundle-status

logging event spanning-tree

logging event status

end

interface Port-channel3

description bciszgr10 po3

switchport access vlan 42

switchport mode access

logging event trunk-status

logging event bundle-status

logging event spanning-tree

logging event status

end

VIP Purple

Host flapping every 2 hours for 15 minutes

Hi Andreas,

You are using both port channel for one server ???

if yes then which server ???

Regards

New Member

Host flapping every 2 hours for 15 minutes

Hi,

Po2 is directly connected to our Backboneswitch where the SVI 42 with the MAC-Address is configured.

Po3 is connected to another Switch. This one has a connection over a L3-Port to the Backbone too.

Regards

Andreas

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