standard time for spanning tree/mac learning

[carl_townshend]

Hi all, when I plug a new pc in my switch, how long does it take for it to come on, and is this learning the mac address or just blocking for spanning tree ?

[farkascsgy]

Hi,

If you use 802.1d ( traditional STP) it takes 50 sec, but if you use the portfast feature (spanning-tree portfast interface command) it put immediately (1-2sec) forward state your port.

The 50 sec comes from:

0-20 sec Blocked

20-35 sec Listening

35-50 sec Learning

50- sec Forwarding

bye

FCS

Please rate me if I helped.

[mohamed.fofana]

Hi Carl,

It depends. If your Cisco switch port (interface) is configured with the spanning-tree portfast command then, you pc will immediately connect to the switch network and be able to communicate but if portfast is not configured on the interface then, plugging the pc into an interface on the switch will trigger a topology change notification (TCN) in an 802.1D spanning tree which will go through the listening (15 seconds), learning (15 seconds: mac address learning) and then forwarding. In this situation, if the pc is getting its ip address from a DHCP server on the network, it will timeout resulting in the pc not obtaining an ip address from the DHCP server.

It is always a good practice nowadays to configure the switch port (interface) with the portfast command. This alleviate a TCN from being sent every time a pc is plugged /unplugged on the switch interface or turn up / shutdown.

I hope this help.

Mohamed