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New Member

Why some switch ports does not show mac address

                   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

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Why some switch ports does not show mac address

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

21 REPLIES
Cisco Employee

Why some switch ports does not show mac address

Hi Mahesh,

What are the switch model and IOS version?

Kind Regards,
Ivan

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

Kind Regards, Ivan Shirshin **Please grade this post if you find it useful.
New Member

Why some switch ports does not show mac address

Hi Ivan

cisco WS-C4506  and

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

Thanks

Cisco Employee

Why some switch ports does not show mac address

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.

Kind Regards, Ivan Shirshin **Please grade this post if you find it useful.
New Member

Why some switch ports does not show mac address

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

Hall of Fame Super Silver

Re: Why some switch ports does not show mac address

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

New Member

Why some switch ports does not show mac address

Hi Giuseppe,

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

Regards

MAhesh

New Member

Re: Why some switch ports does not show mac address

Hello! can you pls elaborate on this "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)"

 

how did you figure out that it was an incompatibility? I'm just working through some issues with devices on a newly installed 4500. Any info would be appreciated 

Why some switch ports does not show mac address

"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

New Member

Why some switch ports does not show mac address

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

VIP Super Bronze

Why some switch ports does not show mac address

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

New Member

Why some switch ports does not show mac address

Hi Reza,

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

Thanks

MAhesh

VIP Super Bronze

Why some switch ports does not show mac address

Hi Mahesh,

Try clearing the MAC table:

clear mac address-table dynamic interface gi2/23

and than try "sh mac address-table again

HTH

New Member

This really helped me, thanks

This really helped me, thanks! I had trouble with a similar device.

The switch didn't cached the mac address of the device. So I just pinged the device :)

 

I think you could ping to the broadcast if you don't know the exact address.

Hall of Fame Super Gold

Why some switch ports does not show mac address

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.

New Member

Why some switch ports does not show mac address

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

Hall of Fame Super Gold

Why some switch ports does not show mac address

How can i find the broadcast IP for each vlan?

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

New Member

Why some switch ports does not show mac address

Hi leolaohoo,

Here is info

  Internet address is 192.168.20.1/24

Thanks

MAhesh

Hall of Fame Super Gold

Why some switch ports does not show mac address

  Internet address is 192.168.20.1/24

Your broadcast IP address is 192.168.20.255.  Ping that.

New Member

Why some switch ports does not show mac address

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

Hall of Fame Super Gold

Why some switch ports does not show mac address

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.

New Member

Why some switch ports does not show mac address

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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