switch1#show interfaces descri
Fa0/13 up up
Fa0/14 down down
Fa0/15 up up

(the lights are also on on both ports 13 and 15)

switch1#show mac-address-table
24 x.dc15.93a9 DYNAMIC Fa0/13
24 x.9931.296d DYNAMIC Fa0/11
24 x.2196.cf24 DYNAMIC Gi0/1
24 x.a947.fb81 DYNAMIC Fa0/11
24 x.b954.90c2 DYNAMIC Fa0/8
24 x.b976.a45c DYNAMIC Fa0/2
24 x.8f1c.f11b DYNAMIC Fa0/11
24 x.ba8e.f467 DYNAMIC Fa0/11
28 x.2196.cf24 DYNAMIC Gi0/1
28 x.f08a.6025 DYNAMIC Fa0/20
28 x.f08a.6026 DYNAMIC Fa0/19
28 x.f08a.6027 DYNAMIC Fa0/22
21 x.2196.cf24 DYNAMIC Gi0/1
20 x.2196.cf24 DYNAMIC Gi0/1
i.e. nothing for port 15! How to determine the MAC address of this port or how to wake up the device? Once again, the lights on both ports are green.


You could drop the entire MAC table and allow the device to 're-learn' all connected devices (should happen on the first received packet)

clear mac-address-table dynamic


If the device connected to port 15 does not send data due to it being "asleep" then bring the port down then back up. The re-detection of the link maybe enough to cause an IP communication to occur.

  • 1
    I'm not sure how this would help. If the device was transmitting at all the mac would already be in the table. Clearing the table won't cause the device to transmit anything. This is typically done when you move a switch, router or host from one port to another and want to quickly update the mac table with the new info to keep downtime to a minimum. – Paul Ackerman Feb 7 '12 at 12:58

The mac-address-table ages out - so if the device (e.g. a pc in standby) is connected, but has not been actively transmitting lately - that's exact the thing to expect to see... Maybe you have the macs noted down someplace, otherwise you will have little chance of success... tsg

  • how to send ping/wol/rubbish from switch down to the port? – Alex Dec 7 '11 at 10:26
  • by the way, I re-plugged both 13 and 15, then ran show int stats - both ports have Pkts Out and Chars Out values but port 15 lower values. Port 13 has mac but port 15 still nothing. – Alex Dec 7 '11 at 10:30
  • Just request a new IP address (with dhcp) and/or ping the gateway. That will make populate the MAC address table. – cstamas Dec 7 '11 at 14:13
  • 1
    If the device on Fa0/15 has a locally-administered MAC address, the switch will not store it in the mac-address-table (ref: en.wikipedia.org/wiki/MAC_Address). Since you are able to get it to transmit when you unplug/replug, you might be able to learn the MAC by setting up a SPAN/monitor session and sniffing the port while you plug it in. – James Sneeringer Dec 7 '11 at 16:11

This issue has been around for quite some time. There are only 2 ways I have been able to accomplish this in the past.

1) If you can take the port down, shutdown the port and wait about a minute then do a no shut. This will usually cause the device to send some DHCP traffic or at least some browser traffic if statically configured.

2) If the device refuses to speak when the link comes up or you don't want to take the link down, figure out what vlan the port is in and ping from a device in the same subnet. This will send a ping to every device in the broadcast domain. If the device is alive, it should respond and thus provide its mac to the switch.

If this doesn't work, you probably have a PC that is connected but turned off and is therefore not sending any traffic and does not have an IP and will not respond to a broadcast ping. In this case, I don't know of a way to get the device's mac. :-(

  • I wonder if there is a Cisco command to broadcast or just send a "dummy" packet to all or some specific ports. Sort of a wake up call up to the cable of connected port(s). – Alex Jan 23 '15 at 4:52

You can get the Mac address of a specific port by using following command:

show interface fastEthernet 0/15

To update the MAC address table, you may require to ping the IP address of the device connected with port 15 of the switch.

  • the device has no IP yet, how to do broadcast arp ping? – Alex Dec 7 '11 at 10:33

Actually what you can do is enable port security on your switch ports for a short time while you pull the MAC address info you need. Then you can turn off port security if you don't need it.

  • 1
    How does port security help determine the MAC address of a device, by the way? – HopelessN00b Oct 25 '12 at 13:48

You might try setting the port, fa0/15 to

switchport mode trunk

temporarily. Not sure if this will work.

Make sure you set the port back to access.

You might also try

int fa0/15
no shut
  • 1
    Scary.. This might break all sorts of things. – Tom O'Connor Dec 7 '11 at 18:17
  • Why would you think that? I have, by mistake, set access ports to trunk ports, and nothing has broken. Have you done it and broke something? – dbasnett Dec 7 '11 at 18:33

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