Aging Time

[dyep110581]

Hi,

I would like to ask What is Aging Time and its purpose.

Thanks,

Dyep

[Joseph W. Doherty · Accepted solution]

Within network devices, there are some functions that acquire information. When they acquire it, they hold on to it so they don't have to reacquire it. They will also usually keep track of how old the information is, its age, and when it becomes too old, it will be discarded. An example would be ARP.

[Giuseppe Larosa · Accepted solution]

Hello Dyep,

the aging time is the timer that decides how long a non speaking MAC address is stored in the CAM table before purging it

the CAM table is the table where the associations MAC address, Vlan, port are stored.

the lan switch learns from user traffic out what port a mac address is looking at source MAC address of frames. But if a host doesn't transmit anything for 300 seconds (this is the default aging time value) its entry is removed. If the host starts again to transmit the entry is created again.

The CAM table has a finite size in the order of 8 thousands for small switches up to 65,535 for C6500.

Hope to help

Giuseppe

[Joseph W. Doherty · Accepted solution]

For the original poster, although Giuseppe provides an excellent description of the MAC aging timer in reference to CAMs, and since you mark his post as resolving your question, I just want to re-emphise in reference to your original question there are multiple aging timers, a couple besides ARP's and CAM's. The principle is generally the same, and such timers might be adjusted for various reasons. (For example, setting ARP and CAM aging timers can make or break unicast flooding.)

[edit]

I wrote too soon, see you've marked my original post as also resolving your question. BTW: The interplay between ARP and MAC aging timer, and how it can cause unicast flooding, I think, is worth knowing. You might want to read http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml

[jlwomeld]

Question; If I have my 6500 arp set global to 300, and a 3750 vlan set to mac-aging set to 14400, could this cause flooding?

Thanks

[Joseph W. Doherty]

I believe the issue is caused by the ARP and CAM MAC timers on the same device, not different devices. Also, with unicast flooding, the return path needs to be different from the original path.

[walebuster@gmail.com]

In fact you have the best and simple answer to the aging time,

Thank you.