Cisco Portfast Configuration

Lessons Discussion
42

Hello Cory

Ah, I see, sorry about that, I misread your post. Yes, if you have a Wireless Access Point connected to a trunk, then the only way that a TCN would be generated is if the actual interface goes down. This would result from either the AP powering down, the cable being cut or unplugged, or the switchport being shutdown.

The above is true assuming that the AP is serving end users such as smartphones, tablets, and laptops. If however that AP is actually acting as a wireless bridge connecting to another switch on the other end of the link, then the behavior may be different.

Remember, in classic STP, a TCN will only be generated if a link failure takes place along the active path to the root bridge. If BPDUs are being exchanged across that wireless link, and all of a sudden that link fails, then that would indeed trigger a TCN on the local switch.

Take a look at this NetworkLessons note on what a topology change is and how a TCN is generated. Also, take a look at this note on the topology change process.

I hope this has been helpful!

Laz

43

why we should not configure porfast on ports connected to FWs Servers etc?

44

Hello Muayad

The rule of thumb is, if you have a single host on a switchport, portfast can be enabled safely. If your port connects to a switch, you should never enable portfast.

Now for servers, firewalls, routers, and any device with a Layer 3 interface connected to the switch (i.e. whose interface is configured with an IP address), it is preferable to keep portfast enabled. Why? Because these devices will unlikely be the source of a Layer 2 loop, and secondly, if they are rebooted, or if you disconnect them for maintenance, when they are connected again, you would want them to immediately begin operating Does that make sense?

I hope this has been helpful!

Laz