Sunday, December 2, 2007

Optimizing Layer2 Switching Environments (Part 1): Spanning-Tree Management

MAC Address Aging
For the stability of a Layer2 switched environment it is important to tune certain parameters. This Part1 will cover the mac address aging timers. The switch can operate in a default "out-of-the-box" configuration, but that won't bring the benefits and expected performance.

Lets say you have a fully redundant switched configuration but you notice that when a link failure occurs some devices are delayed too much to regain connectivity while others almost does not experience the failure. What might be the reason?

The default mac-address aging, for dynamically learned addresses, on a switch, is 5 minutes or 300 seconds. In a fully switched environment, the spanning-tree reconfiguration, might cause for certain devices to become unreachable for up to 5 minutes.

To accelerate the mac address aging you should adjust the "aging-time" parameter under the mac-address-table configuration. This will force the switch to 'forget' about mac addresses who haven't been seen after the pre-configured amount of time. The command to adjust the "aging-time" timer is the following:

mac-address-table aging-time 10

! -- or --


mac-address-table aging-time 10 vlan 100
As you may notice, this is a global command or a "per-vlan" command. Meaning, the mac address aging is accelerated on a per-VLAN basis or per chassis basis.

Now, if your network has too many switches you should also consider what will be the best spanning-tree protocol to operate instead of manually tweaking all the timers. But this will be covered in Part2 of this series.

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