Cisco STP forward-time command trick

This is more a topic for exams, like Cisco CCIE, but also it can appear in real-world environments.

Consider the following simple topology:
Simple L2 topology

SW1 is the spanning-tree root bridge for all VLANs. Imagine that you have a request which ask you that when a port becomes active, no matter of VLAN, it should wait 10 seconds until it transition to forwarding state.

You look into configuration mode and spot the “spanning-tree .. forward-time..” command. OK, simple enough, you go there and type:

spanning-tree vlan 1-4094 forward-time 10

Task completed. This is what I also thought. Looks pretty straight forward, but it’s a mistake which I learned from and I hope you will read this before doing it.
Lucky for me it was just a test lab not the real CCIE lab, but if I don’t teach this kind of things or spend the entire day fine-tuning STP, I tend to forget.

Back to this example, let’s see on the SW2 what’s happening after I did configure the above command on SW1. By the way, you need to configure this command only on the STP root bridge because the downstream devices will inherit the values.

On the SW2 I shutdown the root interface to force the alternate interface (which is in blocking state) to transition to forward state and check the output of “debug spanning-tree events” :

00:55:15: STP: VLAN0001 new root port Fa0/2, cost 19
00:55:15: STP: VLAN0001 Fa0/2 -> listening
...
00:55:25: STP: VLAN0001 Fa0/2 -> learning
...
00:55:35: STP: VLAN0001 sent Topology Change Notice on Fa0/2
00:55:35: STP: VLAN0001 Fa0/2 -> forwarding

From :15 seconds when the port entered Listening state until :35 seconds when it went into Forwarding state there are 20 seconds. We were required to have 10 seconds not 20. That’s not good.

Going back and checking the parameters of a STP for a particular VLAN did not give me too much information to clarify the issue:

show spanning-tree vlan 1 | i Forward
Hello Time   3 sec  Max Age 10 sec  Forward Delay 10 sec

OK, after reading again carefully the documentation I came to the understanding that value added to this command is applied to each state towards the forwarding state.
The STP Port States are: Blocking, Listening, Learning, Forwarding and Disabled.

When I did shutdown the root port, the alternate port when immediately to Listening state then waited for 10 seconds until it transition to Learning state and again 10 Seconds to achieve Forwarding state.

The correct solution was obvious now to decrease the forward-time to 5 seconds.

spanning-tree vlan 1-4094 forward-time 5

Let’s check again:

01:11:55: STP: VLAN0001 new root port Fa0/2, cost 19
01:11:55: STP: VLAN0001 Fa0/2 -> listening
...
01:12:00: STP: VLAN0001 Fa0/2 -> learning
...
01:12:05: STP: VLAN0001 sent Topology Change Notice on Fa0/2
01:12:05: STP: VLAN0001 Fa0/2 -> forwarding

Now the total time is 10 second.

I hope this will help others to avoid my mistakes.


4 thoughts on “Cisco STP forward-time command trick”

    1. Thank you for your advice, but I tend not to completely agree with you. If you read my blog you’ll see that I deal with old and new technology, basically everything I find interesting.
      Then depending on professional position one can still configure a lot of STP. Last but not least, if you want to go for a certification, you need to know STP, the same you need to know Frame-Relay even if there is very less use of it.

  1. Calin,

    Nowadays engineers just use brain dumps and pass all the exams. Life goes to fast to study old and not so useful stuff…

    1. Everybody is entitled to act as they want :) I like to know what I’m talking about, not only have the certification papers. I understand your opinion, but I have a different view of what means to be a “network engineer”.

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