default aging time for MAC address

[hanwucisco]

I am trying to understand this sentence

"A topology change occirs in an STP environment will cause the default aging time for MAC address entries to be reduced for a period of the max age timer plus forward delay interval."

Here, what is a "topology change"? I have two switches set up connected with a crossover cable with vlan 1 on both sides. I shutdown the port from one switch.

and did a show mac address table time. it never changed. it is 300 seconds all the time.

is my understanding wrong?

thanks,

Han

[Peter Paluch · Accepted solution]

Hello Han,

These two events are considered as topology change events in legacy STP:

• A previously Forwarding port going down (i.e. becoming blocked)
• A previously blocked port becoming Forwarding

If you want to verify the MAC aging time, first of all, you have to check for the output of the show spanning-tree command - the other output of show mac address-table may not display the temporarily reduced aging time. Also, I suggest you leave the inter-switch connection running, and instead, connect or disconnect some other port going to a PC.

The statement you wrote about the reduction of MAC aging time is not entirely precise. During the topology change period, the aging time will be redoced to the forward_delay time, i.e. 15 seconds by default. However, the entire topology change period, i.e. the how long the MAC aging time remains decreased, is max_age+forward_delay.

Please have a look at the following document:

https://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml

Best regards,

Peter