STP and uplinkfast

Unanswered Question
Apr 7th, 2008

Hello, I'm trying to figure out if uplinkfast will have any positive effects on my network(stp reconvergence) if we already weight our vlans across two core routers? See the visio jpg for a visual.

From my understanding, uplinkfast allows the protocol to pre-determine the link to forward should the primary link go down which cuts down on recalculation time. Just need to know if it will still help with the scenario I've drawn out in the jpg file? I've heard no, but would like more input.

Thanks in advance.


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Istvan_Rabai Mon, 04/07/2008 - 09:09

Hi Robert,

I saw your drawing.

This the classical config to use uplinkfast.

It will shorten switchover as I said in my previous post.

If you need help with configuring uplinkfast, please tell us and we will help.



Istvan_Rabai Mon, 04/07/2008 - 09:23

By the way,

Why don't you use rapid spanning-tree instead?

What are the switch types you are using? Do they support RSTP?



Istvan_Rabai Mon, 04/07/2008 - 09:06

Hi Robert,

You forgot to post your drawing.

Uplinkfast will decrease the switchover to the alternate root port. Switchover time is a few seconds max.

Without uplinkfast the switchover time is 30 to 50 seconds, depending on when and how the switch notices the absence of its root port.



rsamuel708 Mon, 04/07/2008 - 09:29

Istvan, thank you for your quick reply. I'm new to the organization where this config is used so I'm wondering what weighting the vlans(8192/16384) does for you?



rsamuel708 Mon, 04/07/2008 - 11:00

Istvan, thank you for the information. I think I've got it now regarding the vlan weighting.


Istvan_Rabai Mon, 04/07/2008 - 21:01

Hi Robert,

I believe the weighting you are talking about is the spanning-tree priority of the switches.

It is used to select the root of the spanning tree in a predictive manner.

The switch with the lowest priority value (8192 in this case) will become the root switch.

The 16384 is the priority of the secondary root switch. This is needed to predictively select a redundant root switch in case the primary root switch fails.

This config will work if all the other switches have the default priority configuration i.e. 32768.




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