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Why and how to configure Portfast.

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We all know the big advantage of configuring Portfast, a port configured with Portfast will immediately start transmitting data in the ‘forwarding’ state bypassing the other spanning-tree states. This is certainly a great feature to have configured on your downstream ports connecting to your end-user workstation or your servers. There is also another great reason to configure Portfast on your client edge ports, that is not such widely known.

Whenever a switchport goes up or down the switch generates a TCN (Topology Change Notification) packet and sends this TCN packet to the root bridge, the root bridge then responds back with a TCA (Topology Change Acknowledge) packet simply to acknowledge the TCN packet. The root bridge then transmits another BPDU with the TC (Topology Change) bit set to every switch within the Spanning-Tree domain. When the other switches receive this TC marked packet it resets the aging time of every entry in the CAM table (Also known as the MAC address table) down to 15 seconds which can cause the switch to rebuilt it’s it CAM table if the entries start aging out. Now depending on the size of your layer 2 network this can waste a lot of resources on your switches. Not to mention causes a lot of unnecessary traffic overhead, since we have a set of BPDUs transmitted with the TCN, TCA, and TC flags set individually. Keep in mind also, that if CAM table entries start expiring this can cause unnecessary ARP traffic for additional information the switch already had.

Now let’s review some of this:

Here is a port configuration, without portfast:

Spanning-Tree  Port config

When we disconnect and reconnect fa1/0/5, we get the following log output:

Spanning-Tree TCN with  no Portfast

In that previous screen shot, you’ll notice the first thing that happens is spanning-tree sends out that TCN BPDU, and interface is marked as down. I then reconnect the cable to fa1/0/5 and you see the port go through the spanning-tree stages, from listening to learning and finally to forwarding. (Extra credit which version of spanning am I running?)  Something to notice as well is the fact another TCN BPDU packet is sent as the port is set back in the forwarding state.

Now let’s configure Portfast on this switchport:

Spanning-Tree  Port config portfast

Now again lets disconnect and reconnect this port again:

Spanning-Tree TCN with Portfast

There is much less going on here compared to our previous experience, the important thing here is to notice there are no TCN transmitted not when the port is marked as down nor when the port marked as up (or forwarding). The only STP events that are registered is the fact port fa1/0/5 goes directly to the forwarding state from the blocking state bypassing the listening state and the learning state, allowing the client to start using the network even quicker.

Now, let’s sit back in our chair for a second and think about this for a minute. A TCN is sent out when the switchport goes down and again when the switchport enters the forwarding state. So when an end user decides to reboot their PC, when they undock their laptop to go to a meeting, dock back in at their desk again, or when they decide to re-organize their desk and unplug their PC a TCN wills be transmitted causing the switches to lower the aging timers of entries in the MAC address table. That could be a lot of unnecessary resource utilization.

P.S. Remember to enable BPDU Guard when enabling portfast! Portfast is great a tool but because it skips the listening and learning states you run the possibility of create layer 2 switching loops if cross connect multiple switches or your users start connecting simple switches/hubs at their desks. BPDU Guard will place a port in an Err-Disabled status if it receives a BPDU on that port.

Written by Stephen J. Occhiogrosso

March 28, 2013 at 9:57 AM

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