To portfast or not so fast?

| Thursday, March 25th, 2010 | No Comments »

Cisco’s portfast feature, which later became an implemented feature in RSTP (802.1w) is an interesting feature. It’s purpose is to bring an interfaces state to a “Forward” state as quickly as possible. This, if you are using multiple switches and spanning-tree is being utilized. But what if you’re not? How quickly does the interface go into a forwarding state?

Using a 2960G switch, I first plugged a PC into port G0/3. While pinging my router I went ahead and shut off the interface.

shutdown

Then quickly enabled it again.

no shut

While using a stop watch, it took 30 seconds for the interface to come up, and allow my PC to ping again. 30 whole seconds! If you have a newer PC, or a PS3, Nintendo Wii, Xbox 360, these devices would not be able to get online, and would think there was a network problem. Maybe not so critical for your gaming consoles, but your PC, if it were using DHCP, would not get an IP address for you to log into your domain.

In any event, now I wanted to enable portfast on the interface to see how quickly it goes from disabled to enabled (forward state).

spanning-tree portfast

shutdown

While continuously pinging, I can now see that the pings no longer work.

no shut

Quickly enabling the interface, the pings were able to get through in just a matter of 2 seconds!

That’s fast enough. And it proves that portfast is truly fast! It does put the interface in a forwarding state rather quickly! So now you know!

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