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MAC ADDRESS , Spanning-tree portfast


I have one cisco 2960 switch .

On that switch interface gi0/8 is connected to device. But when i want to show the mac address connected on that port ii cant found it

Step 1>

SH int status


Step 2 >

mac add.JPG

How i can found that port connected device mac add.

And other question is

If i configured  "Spanning-tree portfast " what the status of it also got portfast  or not..

9 Replies 9

Rolf Fischer


mac address entries age out after 5 minutes if on frame with this mac address as source is received on that port.

If the connected device is a workstation which normally operates at 100 mbps, the a-10 could be an indication for a  "sleep mode".

There are some ways to gather information about the devices connected to the edge ports, like network management applications (e.g. CiscoWorks LMS, Netdisco, etc.), DHCP snooping or port security.

The STP portfast command in global mode applies to access- ("non-trunking") ports only:

Hope that helps.


I couldn't get you can u explain in simple words to find the mac-address of that connected port 

And other questin also there

When i configured Spaning-tree port fast globaly...  So what is the status of uplink and hub connected to that port

I want to know spaning-tree chose there own that which port stay in portfast or which not....I mean uplink stay in normal mode or portfast

Your screeshot "step 2" shows the output of the show mac address-table interface gig0/8 command.

This mac address table (sometimes called CAM table) is dynamic.

If a frame is received on a port, the source MAC address, the VLAN and the port will be stored in this table, so frames destined to that MAC address can be send over that port.

If no frame with this MAC address as source is received by the switch for 5 minutes, the entry in the MAC address table is deleted. Now frames destined to that MAC address have to be flooded because there's no entry telling the switch which port to use.

If you use the show mac address-table command and do not get any output, there's no entry for that port because the connected device has been "quiet" for at least 5 minutes.

You could "refresh" the entry by pinging the device (or sending any type of packet which requieres a response), but when you don't know the IP/hostname, that can be a challenge. Or, since the device is obviously not communicating, you could try a shutdown/no shutdown on that port.

If you don't have hundreds of devices in VLAN 1 you could also ping the VLAN 1 broadcast address from a VLAN 1 interface.

There are some IOS features or management applications you can use to collect edge-device information in the future, but this doesn't help you today.

Spanning tree portfast in global configuration mode: It depends on the operation of your uplink port, is it trunking or no trunking? If it's not trunking, it will operate in portfast mode. (I'd recommend to combine it with BPDU guard in this case.)

Hope that helps



To find the mac address if a device connected on a specific port you van issue the command "show mac address-table interface " e.g. if you want to find the mac address of a device connected on fastethernet 0/5 port the command is : show mac address-table interface f0/5.
Regarding spanning-tree portfast command i think you are a littelbit confused. you can not enable spanning-tree portast globally on a switch, You can issue this command just under interface configuration, issuing this command under an interface put the port directly in forwarding state. You can enable or disable globally spanning-tree protocol.
You can find useful information about spanning-tree protocol here :

Hope this helps,

Sent from Cisco Technical Support iPhone App


you can not enable spanning-tree portast globally on a switch,

sorry but I wouldn't agree on this.

spanning-tree portfast (global configuration)

Use the spanning-tree portfast global configuration command to globally enable bridge protocol data unit (BPDU) filtering on Port Fast-enabled interfaces, the BPDU guard feature on Port Fast-enabled interfaces, or the Port Fast feature on all nontrunking interfaces. The BPDU filtering feature prevents the switch interface from sending or receiving BPDUs. The BPDU guard feature puts Port Fast-enabled interfaces that receive BPDUs in an error-disabled state. Use the no form of this command to return to the default settings.

spanning-tree portfast {bpdufilter default | bpduguard default | default}

Best regards


P.S.: I'd recommend the bpduguard default option.

You're right!
I just never used spanning-tree portfast command in global-mode and i ever thought it was not possiible!

Sent from Cisco Technical Support iPhone App


Make sure port and device are in the same vlan

Sent from Cisco Technical Support Android App


What I do in this situation is to first try a ping sweep of the IP addresses in the VLAN since it could be possible the MAC address timed out of the table.  I use nmap for this, however, there are a lot of other applications out there that can perform the same sweep.  If the machine is on, this ping sweep is usually enough to get the MAC to report on the switch.

If the ping sweep doesn't work, it is likely that the machine is configured for Wake On Lan (WOL).

I've seen this happen from time to time on Cisco network devices.

See if you can pign the IP address associated with that host device, then then look at the ARP table, and see if it responds with a MAC address assocaited with that part, as well as look at the mac address table.

If this doesn't work, clear the ARP ARP table and MAC table, and try the above again.

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