Cisco switch: Trunk port MAC address disppearing from MAC address table

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I have users complaining of slowness on their machines but we can't find anything specific (ping tests, etc). I'm trying to rule out the connection between the switch they're plugged into (3560) and the core switch (a stack of 3570s).

Currently the 3560 has one fiber connection trunked back to core switch. When tracing the trunk port back through the network I run into the odd issue of the MAC address of the 3560's trunk port disappearing from the MAC address table in the core switch. In the output below Switch1 is the core switch stack and Switch4 is the switch I'm investigating.

Switch4#sh int tru

Port        Mode             Encapsulation  Status        Native vlan
Gi1/2       on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi1/2       1-4094

Port        Vlans allowed and active in management domain
Gi1/2       1-2,30,40,42,50,60,70,72,80,82,99-100,102,254

Port        Vlans in spanning tree forwarding state and not pruned
Gi1/2       1-2,30,40,42,50,60,70,72,80,82,99-100,102,254

Switch4#sh int gi1/2

GigabitEthernet1/2 is up, line protocol is up (connected) 
  Hardware is Gigabit Ethernet, address is XXXX.dbd3.f0b2 (bia 30e4.dbd3.f0b2)

–Confirming which port is trunked back to Switch1

=======

Switch1#sh mac address-table | inc f0b2
   1    XXXX.dbd3.f0b2    DYNAMIC     Gi2/1/3
Switch1#sh mac address-table | inc f0b2
   1    XXXX.dbd3.f0b2    DYNAMIC     Gi2/1/3

–The mac address shows up when I search for it

Switch1#sh mac address-table | inc f0b2
Switch1#sh mac

–The mac address doesn't show up suddenly

Switch1#sh mac address-table | inc Gi2/1/3
   1    XXXX.dbd3.f0b2    DYNAMIC     Gi2/1/3
   1    XXXX.480e.2751    DYNAMIC     Gi2/1/3
  40    XXXX.11a0.5372    DYNAMIC     Gi2/1/3
  70    XXXX.5f0e.a116    DYNAMIC     Gi2/1/3
  80    XXXX.b6da.d050    DYNAMIC     Gi2/1/3
 100    XXXX.dbd3.f0c1    DYNAMIC     Gi2/1/3

–The mac address shows up in the table now

Switch1#sh mac address-table | inc f0b2   
   1    XXXX.dbd3.f0b2    DYNAMIC     Gi2/1/3

–The mac address shows up when I search for it again

Switch1#sh mac address-table | inc f0b2

–Then it doesn't

Switch1#sh mac address-table | inc Gi2/1/3
   1    XXXX.480e.2751    DYNAMIC     Gi2/1/3
  40    XXXX.11a0.5372    DYNAMIC     Gi2/1/3
  70    XXXX.5f0e.a116    DYNAMIC     Gi2/1/3
  80    XXXX.b6da.d050    DYNAMIC     Gi2/1/3
 100    XXXX.dbd3.f0c1    DYNAMIC     Gi2/1/3

–And it doesn't show up in the mac address table now

Switch1#sh mac address-table | inc f0b2   
   1    XXXX.dbd3.f0b2    DYNAMIC     Gi2/1/3

–Now it's back

Can anyone explain why this behavior is going on? I've checked on a few more trunk ports between my switches and I don't run into the "disappearing mac address" issue.

Best Answer

My guess? It's mac flapping.

Do show logging on the switches to get the error messages first.

This could be a faulty link between the switches or Spanning Tree(STP) issue. There's only one trunk line between switches ?

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