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Webcast-Catalyst9k
New Member

MAC disappearing from "show mac address-table interface"

I have several Catalyst 3550 switches running IOS 12.1(14)EA1. I have observed the

following behavior:

PC connected to port boots, works fine. Output from "show mac address-table interface f0/8" lists the MAC of the PC. If the PC is quiet on the network, as shown by interface packet counters not changing, then after a few minutes the "show mac ..." command produces output with no MAC address listed. This will happen repeatedly, although sometimes the MAC once again appears in the output even though the

packet counters still haven't changed.

Has anyone else seen this? I don't see anything in Bug Tracker about it, but I do

see a few bugs which look a lot like it.

Thanks,

ERM

4 REPLIES
Bronze

Re: MAC disappearing from "show mac address-table interface"

This is normal switch operation. A MAC address ages out of the MAC address table if the switch has not seen any frames sourced from that MAC address before the MAC timer expires. The default value for the MAC timer (I'm not sure if that's the exact name) is 5 minutes. The timer is reset with each frame sourced by the address.

HTH,

Mark

New Member

Re: MAC disappearing from "show mac address-table interface"

I forgot to add that the aging time has been

set to 3600 seconds, and that the behavior is

seen over periods on the order of 200 seconds.

I did mention that the input packet counter

isn't incrementing while this happens, so the

switch isn't receiving packets to cause an update of the MAC table.

ERM

New Member

Re: MAC disappearing from "show mac address-table interface"

Hello,

I have the exactly the same issue and the disappering appends very fast. It appends in C3750 switches, C2960S switches too. My MAC address aging time it 300 sec.

What could cause this kind of behavior ? The pc does'nt move at all... Only one input packet in the switch is supposed to trigger a mac address table entry usually.

I need answers too

Dave

PS: Take a look at this, I've bold the timestamp of my commands to show you what appends within 6 seconds... and I'm PINGING the PC constantly during that period

cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:00
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
350    0011.250d.d739    DYNAMIC     Gi4/0/14
Total Mac Addresses for this criterion: 1
cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:01
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:02
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:03

          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:04

          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
cat3750#show mac address-table int g4/0/14           -------------------- TimeStamp 0:05

          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
350    0011.250d.d739    DYNAMIC     Gi4/0/14          -------------------- TimeStamp 0:06

Total Mac Addresses for this criterion: 1
cat3750#


cat3750#show mac address-table aging-time
Global Aging Time:  300
Vlan    Aging Time
----    ----------
cat3750#show mac address-table aging-time vlan 350
Global Aging Time:  300
Vlan    Aging Time
----    ----------
350     300

cat3750#

Purple

Re: MAC disappearing from "show mac address-table interface"

Hi,

Can you verify if the mac address appears on other port? if you sniff on a pc in same vlan do you see flooding of the ping?

Regards.

Alain.

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