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switch which doesn't learn a mac-address


I've many switch Cisco which have a problem with mac-address. For example, on one switch cisco 2950, when I connect a pc on interface fa0/8, I don't have address mac when I launch the command "show mac-address-table interface fastEthernet 0/8".

The configuration of interface fa0/8 is :

switchport access vlan 10

switchport mode access

Why ?


Hall of Fame Super Silver

Re: switch which doesn't learn a mac-address

Hello Jean,

a transparent bridge like an ethernet LAN switch can learn a MAC address only when the PC NIC speaks on the wire.

If you are using DHCP you can trigger the PC NIC to speak by performing ipconfig / renew in the shell.

After this the switch port should learn the PC's NIC MAC address.

Prerequisite is that the port is up/up so the two ports agree on speed and duplex or after a successful negotation or because they are statically configured in the same way.

Hope to help


Hall of Fame Super Silver

Re: switch which doesn't learn a mac-address

Hello Jean,

another thought is the following: when the switch port comes up it has to go through the STP states of blocked, listening, learning and forwarding

so when you connect a PC to fas0/8 be prepared to wait at least 50 seconds before the port can learn the MAC address and be full operational.

On an access port you can reduce this time by issuing the spanning-tree portfast command

Hope to help


New Member

Re: switch which doesn't learn a mac-address

Is there anything plugged into the ports? Are there nodes plugged in but may not be switched on? Maybe the MAC's have timed out - try pinging from a node on one of switch ports to the DG and then check MAC table again.

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