Spanning Tree Protocol (STP) has been around the block

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Spanning Tree Protocol (STP) has been around the block

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Added by September 7, 2012

Last time we were introduced to the basics of the Spanning Tree Protocol, the loop prevention mechanism that has been with us since 1985. STP results in a network where some ports are transmitting frames and some are blocked (hence the title you see above).  This blog will discuss the default timers of the original 802.1d specification and what that means for your network.  First of all, you are probably not counting on the original default timers in your network, but since it is still fair game on the exam, we do well to focus on the topic.

You have probably heard it stated that the default values configured for 802.1d Spanning Tree result in a maximum network diameter of 7 switches.  The diameter mentioned here is the greatest distance between any two switches in your switch fabric, including the origin and destination switches.  For this to be meaningful, we need to understand the other timing aspects of Spanning Tree.  Without going into excruciating detail, we can attach values to the following items:

max_age        20 seconds

hello time      2 seconds  (Switches send BPDUs every 2 seconds by default)

Now that we have values, we can use the following formula to determine why the maximum recommended diameter of a switched network is seven.

You can scour the internet to find it, but here is the formula we will use:

max_age = (4 * hello) + (2 * dia) – 2

Plugging in the known values gives us:

20 = (4 * 2) + (2 * dia) – 2

Since we are solving for dia (d), let us continue the steps.

20 = 8 + 2d – 2    Combining values gives us

20 = 6 + 2d           Now subtract 6 from both sides

14 = 2d                   Divide both sides by 2

d = 7                       And that is from where the maximum diameter of 7 switches derives.

What does this mean, especially for a modern network?  This means that a topology change which is managed by the original STP could cause an outage of anywhere from 30 to 50 seconds.

In 1985, an outage of 30 seconds was tolerable.  Nowadays, when down time is measured in milliseconds, 30 seconds is an eternity.  If you are a large corporation doing huge online business, 30 seconds could cost thousands, hundreds of thousands, or even millions of dollars.  Maybe ‘millions’ is an extreme example but it serves to illustrate the point that if a company stands to lose a lot of money during an outage, it’s worth it to spend a lot of money to attempt to ensure that an outage does not occur.  It also is to the benefit of a network admin to have a grasp of the concept of loop prevention in a switch environment.  Anyone can plug in a switch and connect it to a network.  It takes a well-trained admin to fine tune and fully understand the consequences of resiliency and redundancy while avoiding switch loops.

In a future blog, we can discuss how the introduction of Rapid Spanning Tree Protocol has reduced this outage window to a more manageable time.

I have just glanced off the surface of this topic.  If you are one who enjoys the intricacies of mathematics, I recommend reading Understanding and Tuning Spanning Tree Protocol Timers on Cisco’s website. There you will find a much deeper coverage of the formulae involved in computing every little nuance of STP.


Mark Jacob
Cisco Instructor – Interface Technical Training
Phoenix, AZ

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