STP - Changing root bridge

[Ehsan M.]

Hi guys,

very quick question:

I have all the ports in my switches in "Forwarding" stats and basically there are no redundant paths to any switches. So ther are no blocking ports by STP.

So far all is good no problem. Ok I now need to change the root bridge for one of my VLANs to another switch. I was wondering is it going to introduce downtime for traffic for that particuler VLAN (i.e. VLAN 110) due to calucation required by STP? I need to know it now because if that's the case and there will be downtome, I would have to schadule a manitenance window to change the root bridge for this vlan.

Thanks,

Ehsan

[Giuseppe Larosa · Accepted solution]

Hello Ehsan,

>>  I was wondering is it going to introduce downtime for traffic for that particuler VLAN (i.e. VLAN 110) due to calucation required by STP

yes consider roughly 1 minute, it is better to work under a maintenance time window for your own safety

Hope to help

Giuseppe

[sachinsharma2010 · Accepted solution]

yes ofcourse it will take 300 seconds to again select the root bridge and till that time that port will be go in in blocking stage.

[Peter Paluch · Accepted solution]

Hello,

300 seconds? You are probably confusing this with the MAC address table aging time. A new root bridge will be recognized in at most 20 seconds (the max_age timer). The change of a root bridge may put existing ports back into Listening state, requiring them to go over the Listening -> Learning -> Forwarding sequence. So in the worst case, the outage should last around 50 seconds, roughly a minute as Giuseppe explained.

Best regards,

Peter

[kert_dezcom]

If you don't have redundant stp links you don't need scheduled downtime. If you have - aprox 45 seconds to elect new root bridge.

[Peter Paluch]

Hello Alex,

I beg to differ. Even in network with no redundancy, a change to STP root will cause temporary outage of the network. If a switch detects a change in the root bridge, it will basically reset its STP and start it anew, i.e. it will need to select a new root port, new designated ports, etc. - with each port needing to go through the Listening->Learning->Forwarding sequence, resulting in guaranteed outage of 30 to 50 seconds. In addition, how did you compute the 45 second delay?

Best regards,

Peter

[kert_dezcom]

Hello, Peter

Sorry my wrong calculations ~ 50 sec (max age + 2xforward delay) with default timers, you right.

but I tried to change root bridge from one switch to another and i've got no outage...

.

[Peter Paluch]

Hi Alex,

Are you perhaps running Rapid STP or BackboneFast/UplinkFast? These features would modify the default STP behavior.

Best regards,

Peter