Timeout for mac address table in Cisco 3750E

[more_jazZz_2]

Hi All,

I have a simple question. Please suggest the correct answer.

I have 3750E swiches with IOS v. 12.2(55)SE3. Couple servers connect to ths switch, after ping of IP this servers I can see mac addresses in mac address table.

For instance:

sw1#show mac address-table vlan 20

          Mac Address Table

-------------------------------------------

Vlan    Mac Address       Type        Ports

----    -----------       --------    -----

  20    e41f.13b0.e809    DYNAMIC     Gi1/0/23

  20    e41f.13b2.5b71    DYNAMIC     Gi1/0/20

  20    e41f.13bd.e808    DYNAMIC     Gi1/0/17

After ten seconds when I try to see this mac addresses again it absent.

Aging time is default:

sw1#show mac address-table aging-time vlan 20

Global Aging Time:  300

Vlan    Aging Time

----    ----------

  20     300

When ip generate traffic to this server this mac address appear in table again on shot time (less than 10 seconds).

Anybody knows What happens with mac address table in switch?

Zak.

[rsimoni · Accepted solution]

Hi Zak,

mac addresses can disappear from CAM table if MAC table gets flushed. It gets fushed when STP TC's (Topology Change) occur following network reconvergence/link flappings.

You can check TCN number and how often you see them in vlan 20 with the following command:

lan-3750-1#sh span vlan 99 det

VLAN0099 is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 99, address 0024.5043.c700

  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 22w6d ago  <<<<<<

          from GigabitEthernet1/0/20

  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   

Of course you need to check that in vlan 20.

Take the outputs every 30-45 seconds and check the total number of TC and the port which triggered it. Then you follow that port until you find the flapping link or the switch which keep on sending them.

regards,

Riccardo

[more_jazZz_2]

Riccardo, thank you for helping. I have found this ports and problem resolved.

Thank you!

Zak.

[more_jazZz_2]

Iit was simple server without operation system and ethernet ports in this server constantly up and down. I configured portfast globaly for access ports and problem resolved.

Thank you!)