root bridge election

[PooriGoudarzi35482]

Hi to all

i want to be noticed that what happens when root bridge for a (V-LAN) fails and new root bridge elects what are the consequences in live network?

I mean do we face link up/down and convergence time on the root port or the designated ports that are the servers????(the port fast is not enabled on the ports that are connected to servers)

thanks

[Joseph W. Doherty]

If I remember correctly, the STP topology is (logically) blocked until the topology converges with the new root.  Physical links do not go down, but interconnecting topology links will not forward "normal" traffic.

As to edge ports having port fast enabled or not, I don't think that matters, but not 100% positive.

What I also don't recall is whether hosts on the same switch are blocked against other hosts on the same switch.

I also recall re-convergence with new root is generally rapid with rapid-STP (or MST) but can take a while with non-rapid-STP.

[MHM Cisco World]

We can use the feature called Spanning Tree Protocol (STP) PortFast to speed up convergence on ports which are connected to a workstation, a network printer or a server (which are end devices and cannot cause layer 2 loops). PortFast feature should be used only to connect a single workstation to a switch port to avoid l2 loop. Spanning Tree PortFast feature causes a port to enter the forwarding state immediately, bypassing the listening and learning states.

see that the port-fast will make the port go to forward and reduce the coverage time. 

[Martin L]

When STP Root Sw fails, new root bridge election happens.  All switches will clear MAC addresses and learn them again if there is traffic which is not a problem at all - a less then 1 second to clear and learn MAC again; 

Depending on location of switch and topology, you may have between 30 - 51 seconds traffic disruption while ports re-learn their roles.  PortFast takes 2x15 seconds away from those times on edge ports, so it is recommended to have it enabled even for trunking ports going to servers (portfast trunk).  Note that STP BPDUs flow is from Root sw but not data traffic. Data traffic does not have to go via Root sw.  So, if 2 hosts communicate via the same switch -traffic never goes over trunk link- there will be no noticeable loss.

Regards, ML
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