STP question

[joan1971]

Hello,

I have a question: In what case it is OK to manually modify STP path costs in a BPS2000 or a Baystack 380 switch? And if path costs are manually modified, what are the repercussions on  STP computation?

Thanks,
Joan.

[Flintstone]

Hi joan1971,

It is OK to modify STP path cost if you want your traffic to go via a preferred route and not be blocked on that port.  The STP computation we be as normal, but when your switch decides on which port to block then port costs, priority etc will be taken into consideration on deciding which port to block.

CheerZ

[joan1971]

Hello Flintstone!

Thank you for your answer. It took a while for me to read it.. I was a little sick, I'm OK now!

So If you configure all ports in a switch with the same path cost value, STP will do what it has to do anyway. I know it sounds like a dumb question, but changing all ports' path cost to 100  is what the support team is doing to our network in order to solve this problem: http://forums.networkinfrastructure.info/nortel-ethernet-switching/broadcast-flood-help!/
I am certain that it won't change anything but I thought I should give them the benefit of the doubt and ask you guys (that I trust better!) if there is some Nortel/Avaya special trick with spanning tree path cost calculation that I am not aware of (and of which you would definitely be, if there was such a thing)!

Thanks again,
Joan.

[Dominik]

Hi joan,

in general you can difine your own values for the spanning tree path costs.
The normal method is to define standrds for your network and configure all path cost to that values.
The important thing here is that all links in your network are configured consistent.

The most common configuration was in the past to use these values depending on the Port speed:
Port speed     path cost
10      Mbit   >> 100
100    Mbit   >> 10
1000+Mbit   >> 1

In fact of that 10Gig and 40Gig Ports becomming more common in modern networks,
this doesnīt work out anymore.


Cheers

[joan1971]

Hi Dominik,

Thank you for your answer.
Some links where set to 1 and the rest where set to 100. We changed them all to 100 but the problem is still here. I noticed that a blocking port remained blocking after changing all links' path cost values on a switch (and those of the "opposite" links), which confirms that STP takes other factors more "under consideration" than path cost when it calculates for preferred route.
Anyway the value 100 is for 10Mbit when our network is 1Gb so it's a wrong value to begin with!

So, now we are waiting for Avaya to diagnose and provide a solution.
I'll let you know what they said!

Thanks again,
Joan.