Why some switch ports does not show mac address

Answered Question
May 23rd, 2012

                   Hi all,

I was finding  out which device is connected to switch port.

I did this command to find mac address

sh mac address-table int gigabitEthernet 2/23

Multicast Entries

vlan    mac address     type    ports

-------+---------------+-------+--------------------------------------------

225    ffff.ffff.ffff   system Gi2/1,Gi2/2,Gi2/4,Gi2/5,Gi2/6,Gi2/7,Gi2/8

                                Gi2/9,Gi2/10,Gi2/11,Gi2/12,Gi2/13,Gi2/14

                                                              Gi4/39,Gi4/43,Gi4/48,Po25

625    ffff.ffff.ffff   system Gi2/1,Gi2/2,Gi2/5,Gi2/6,Gi2/7,Gi2/8,Gi2/9

                                Gi2/10,Gi2/11,Gi2/12,Gi2/14,Gi2/15,Gi2/16

                                                              Gi4/48,Po25

seems ffff is all broadcast address .

Need to know why this is not showing mac address thanks

MAhesh

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 1 year 11 months ago

Hello Mahesh,

L2 information for the port looks like fine.

To learn a MAC address on the port the attached device has to speak and to be understood by the switch port.

So a first possible case is given by some incompatibility between more modern C4500 linecards  (I remember a case with WS-4648 on new C4500 that required upgrade of network drivers on PC NICs)

Another possible situation is given by wake on lan the connected device is powered but it does not show activity so no MAC is learned on the port.

CAM aging time is by default only 300 seconds so another case is a device like a PC that is powered on, connected but it hasn't used the LAN for more then 300 seconds so its entry has expired and it has been removed from CAM table

Hope to help

Giuseppe

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Ivan Shirshin Wed, 05/23/2012 - 09:47

Hi Mahesh,

What are the switch model and IOS version?

Kind Regards,
Ivan

**Please grade this post if you find it useful.

mahesh18 Wed, 05/23/2012 - 10:28

Hi Ivan

cisco WS-C4506  and

bootflash:cat4500-ipbasek9-mz.122-54.SG.bin"

Thanks

Ivan Shirshin Wed, 05/23/2012 - 11:05

Hi Mahesh,

Check if you have broadcast storm in the network. Also verify for STP transitions:

    show span int gi2/23  //check for topology changes

I saw similar issue related to static mac configuration. Do you have any of that in the config?

Kind Regards,
Ivan

**Please grade this post if you find it useful.

mahesh18 Wed, 05/23/2012 - 11:30

Hi,

Here is info

sh spanning-tree int gi2/23

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0225            Desg FWD 3100      128.87   P2p Edge
VLAN0625            Desg FWD 3100      128.87   P2p Edge

I chekced no static mac on switch port config.

thanks

mahesh

Correct Answer
Giuseppe Larosa Wed, 05/23/2012 - 14:33

Hello Mahesh,

L2 information for the port looks like fine.

To learn a MAC address on the port the attached device has to speak and to be understood by the switch port.

So a first possible case is given by some incompatibility between more modern C4500 linecards  (I remember a case with WS-4648 on new C4500 that required upgrade of network drivers on PC NICs)

Another possible situation is given by wake on lan the connected device is powered but it does not show activity so no MAC is learned on the port.

CAM aging time is by default only 300 seconds so another case is a device like a PC that is powered on, connected but it hasn't used the LAN for more then 300 seconds so its entry has expired and it has been removed from CAM table

Hope to help

Giuseppe

mahesh18 Thu, 05/24/2012 - 19:59

Hi Giuseppe,

Thanks again seems its some wake on lan setup on main switch.

Regards

MAhesh

rizwanr74 Wed, 05/23/2012 - 11:53

"Why some switch ports does not show mac address"

Because those port in question in a vlan, for which have layer2 definition yet to be created.

meaning you assigned a port to a vlan but you have not created a layer2 defenition.

Hope that answers your question.

thanks

mahesh18 Wed, 05/23/2012 - 11:59

Hi,

Thanks for reply.

I check port config it is assigned to Voice and data vlan.

Can you please explain below

meaning you assigned a port to a vlan but you have not created a layer2 defenition.

thanks

MAhesh

Reza Sharifi Wed, 05/23/2012 - 14:41

Mahesh,

In addition to the options provided by Giuseppe (Hi Giuseppe, long time no see) try pinging the device and than issue the sh mac address-table command. 

This usually does not apply to PC, Laptop, etc.... but I have seen some video camera encoders that the MAC address does not show up on the switch unless you ping it first.  Strange right?

HTH

mahesh18 Wed, 05/23/2012 - 15:52

Hi Reza,

Sh ip arp does not work also i do not know IP address of device.

Thanks

MAhesh

Reza Sharifi Wed, 05/23/2012 - 16:01

Hi Mahesh,

Try clearing the MAC table:

clear mac address-table dynamic interface gi2/23

and than try "sh mac address-table again

HTH

Leo Laohoo Wed, 05/23/2012 - 15:00

Most of the time, the MAC address does not show up when the clients have logged off.  This is very common nowadays with PCs.  Another is when a server's dual NIC isn't configured properly for "teaming".

If you don't know the exact IP address, there's an old trick (not foolproof) you could try.  IF you know what VLAN the NIC is in, ping the broadcast IP address from the switch.  Sometimes it just may work.

mahesh18 Wed, 05/23/2012 - 15:51

Hi leolaohoo,

The port is up up.

I know port is in voice and data vlan.

How can i find the broadcast IP for each vlan?

Thanks

MAhesh

Leo Laohoo Wed, 05/23/2012 - 16:59
How can i find the broadcast IP for each vlan?

Please post the output to the command:  sh interface VLAN .

mahesh18 Wed, 05/23/2012 - 17:32

Hi leolaohoo,

Here is info

  Internet address is 192.168.20.1/24

Thanks

MAhesh

Leo Laohoo Wed, 05/23/2012 - 18:37
  Internet address is 192.168.20.1/24

Your broadcast IP address is 192.168.20.255.  Ping that.

mahesh18 Wed, 05/23/2012 - 20:36

Hi leolaohoo,

If i ping to broadcast IP  my concern is that will this increase cpu spike as it is production device?

When i ping this IP it will send ping to all devices connected to this Vlan right?

also to other nei switches which are in data vlan right?

Thanks

Mahesh

Leo Laohoo Wed, 05/23/2012 - 20:43
If i ping to broadcast IP  my concern is that will this increase cpu spike as it is production device?

Ping won't kill CPU.

When i ping this IP it will send ping to all devices connected to this Vlan right?

also to other nei switches which are in data vlan right?

If your switches are all in the same subnet, yes.

mahesh18 Thu, 05/24/2012 - 07:49

Hi Leolaohoo,

My bad the switch function as layer 2 it has static default route to core device.

Data vlan is on core switch.

IT has runk connection to core switch.

Thanks

MAhesh

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Posted May 23, 2012 at 9:08 AM
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