MAC address when PC disconnects

[Michael_]

How does a switch handle a frame for a PC when that PC recently disconnected? In other words, the PC's MAC address is already in the MAC table of the switch, and then the PC decides to disconnect, and then moments later a frame arrives destined for that PC. Does the switch automatically remove the table entry for the MAC address upon disconnect, or is there some predetermined wait period?

[Keith Barker]

Hello Michael -

When the switchport loses link (the PC unplugs, or shuts down it's interface), the mac address will be immediately removed from the mac address-table.


Best wishes,
Keith

[Robin Guo]

hehe, keith, i am always study from you.

i favorite it.


Best Regards
Robin Guo

[Michael_]

Keith, that is very helpful. Thanks.

I would like to ask a follow up question...when a PC-A sends a frame to PC-B via a switch, the switch learns the MAC for A and B. However, when PC-A sends a broadcast frame and the switch floods to PC's B, C, and D, why does the switch not learn the MAC's for B, C, and D?


Thanks again. Mike.

[TheOtherTomJones]

MAC addresses are removed from the MAC address table and (I think the ARP table too) after 300 seconds by default (5 minutes).

http://www.ccietalk.com/2008/05/27/chan ... aging-time

https://supportforums.cisco.com/message/53325

[MacM]

Correct me if I'm wrong, but it will learn those mac's for B-D, but only if they respond after the flooding, ie the frame made it to the correct destination and the unlearned PC then sends back a frame with its mac in the source position. At the point in time while the switch is flooding.....the switch still does not know the macs of B-D because those devices have not spoken yet (assuming a lot here), hope that helps. Another good one to learn is what happens when one switchport has a hub connected which has multiple pc's on it, and what happens with the mac table in similar scenarios . -- mac

[Pete2501]

Hey Michael,

Regarding your follow up question. Before the packet is sent an ARP is broadcasted because Host A has no record of the device. Only host B responds to the ARP request and the switch learns host A's MAC address when it receives the ARP request and learns host B's MAC address when it receives the ARP response. Hosts C and D drop the packet because the destination IP address doesn't match so the switch never receives a packet from them.

When a broadcast packet is sent and all devices respond the switch does learn the other host MAC addresses.


Chuck it in packet tracer and test it out.