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Actual impact for users when STP converges

Let's say i have a root/backup root bridge with 15 access switches connected to them, running RPVST+.

 

I then decide to connect a new network to this existing network (also running RPVST+) using one utp link. The new network consists of around the same amount of switches and the stp priorities are higher than the current network.

 

What would a user notice if i at once add this amount of switches via one link?

 

Slowness for a few seconds? Actual network interruption?

 

Everyone's tags (2)
5 REPLIES 5
VIP Advisor

Re: Actual impact for users when STP converges

As long as they are higher priority they not going to be root bridge, then user should not see any network interruption.

But keeping in mind that STP is Evil, if you do not configure properly. so better to show what is your exiting config of both. so we can verify properly rather discussing the theory here.

 

show stp priority of both along with who was the root bridge here.

BB
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Beginner

Re: Actual impact for users when STP converges

If we make sure Root Guard is configured on existing Root switch, any new switch in picture (even with higher priority) should not cause any Root switch change. Isn't it?

Hall of Fame Expert

Re: Actual impact for users when STP converges

Hello ,

as noted by BB if your STP configuration is appropriate there should be minimal impact, just the time in less then one second to perform handshaked of the new link with your root bridge getting designated port status and the other side agrees on this.

 

Note: an effective Rapid PVST configuration requires proper configuration of ALL access ports as edge (portfast).

In this way they will not cause topology changes and what is more important they do no take part in the synchronization wave that happens after any topology change.

Cases have been reported when Rapid PVST convergence was even slower then PVST+ because of not  having configured the access ports as edge (portfast in Cisco terms ).

This is a key point.

 

Hope to help

Giuseppe

 

Re: Actual impact for users when STP converges

Yes the new switches have a garuanteed hardcoded higher priority than the current ones. They will not become the root bridge.

 

I understand that if there's a root bridge change traffic will flow differently having more impact. But what exactly will be the impact then? Would user lose connection perhaps? Or just a long pause?

 

Was just curious what the actual user would notice if there's only a STP calculation. You say impact is minimal, as in a user wouldn't notice a thing?

Hall of Fame Expert

Re: Actual impact for users when STP converges

Hello,

Rapid PVST relies on point to point links between switches where a proposal/ agreement handshake happens between the two devices. Each switch will report its own port current state and the proposed role for the port.

It is just the time to exchange three STP frames on the link without the need to wait for any STP timers.

The switch with the better parameters will become the designated port on the link, the other side agrees on this  and its own port becomes the new root port. This change will propagate on the new added switches as a synchronization wave involving each inter switch link with a root port at one end (one single link involved in new negotiation proposal/handshake per time)

So sub second convergence is possible with Rapid PVST if  is configured correctly.

 

This is the true point of force of Rapid STP, the capability to accomodate a change without the need of going through the listening , learning states anfd their 15 seconds timers when a new link is added to the network.

 

Hope to help

Giuseppe

 

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