CCIE Pursuit Blog

March 19, 2008

STP Diameter

Here’s a nice summary of the STP diameter command from GroupStudy:

Hi Gang,

I have a question regarding the STP diameter command, does it do anything?  like stop BPDU’s or something?  I’ve lab’d this up and it doesn’t appear to do anything significant (if at all).

Mucho appreciated,

D

No, the STP diameter is not a hard limit. It is a recommendation from the RFC. Going over the limit can lead to network instability during convergence, but that does not only depend on STP diameter, but also on the STP timers, the number of MAC addresses, etc…

That is just the problem: try explaining to people that they need to limit the STP diameter. Ooh..we add one switch more and see..it still works.. what’s the problem ?

regards,
Geert

So why is it a configurable parameter, it must do something?  Does it just alter timers to “better” assist in the size of the network?

Exactly.
STP DIAMETER modifies stp timers in the switch to optimize convergence time depending in the size of the SPT DOMAIN.(size of the STP DOMAIN is the number of hops “switches” connected one to the other).

By default the STP timers in every switch are designed to operate well in a domain with 7 HOP switches. (7 hops away from the point of view of the root switch).

Now for example if your domain consist of 3 switches (from the point of view of the root switch), you can modify the diameter to 3, to optimize the stp bpdu convergence time.

By modifying the diameter, the IOS automatically adjusts the hello time, forward delay time and aging time that best fit in a network size of 3 switches.

So the purpose of the tool is to OPTIMIZE the stp convergence time.

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