A new switch tries to be root?

[bbravo]

Can a 2900 switch that is NOT on the same VTP domain but on the same physical network attempt to become a root bridge? How can I check if has become a root bridge?

[scottmac · Accepted solution]

I think you are confusing two different protocols.

VTP is used for management and automatic propagation of VLAN information.

I think you are meaning Spanning-Tree Protocol (STP).

When you introduce an STP -enabled bridge into the segment, it and the other bridges will re-converge ... that is, they will decide which will become the root by comparing several parameters, with "Priority" being the most significant.

If all other parameters being identical, the lowest MAC address will become the Root bridge.

I believe you can verify the root bridge with a "show spanning-tree" command ... there are several options, check 'em out with the "?"

Good Luck

Scott

[bbravo]

Thanks! and would this be a transparent process, meaning the root election reconvergence will not affect normal production of other switches/users in the same STP network if the new switch is manually configured so it will never become a root?

[CriscoSystems]

STP elections take place using information contained in the BPDU's. BPDU's are transmitted every two seconds by default, during normal, production operation.

So yes, root elections and reconvergences are transparent.

Remember to rate helpful posts.

-- stuey

[scottmac]

It may or may not be "felt" on the network, depending on the size and diameter (max number of bridge hops).

Traffic does not pass while all the bridges are figuring out who's the boss ... think of a Cisco switch that doesn't have PortFast enabled ... ~50 seconds while it learns.

With the goal being to not allow parallel concurrent paths (i.e., prevent loops), re-convergence will shut down the segment for *some* period of time while the election of the root takes place.

Back when Microsoft's campus had one, huge, flat network (all NETBEUI/NETBIOS, remember them?), reconvergence was rumored to take, depending on the source, tens-of-minutes to hours to fully occur.

That's why, on large/very large/huge networks, care must be taken to properly integrate the new devices and see to it that all of the active protocols have been configured completely (and to turn off protocols that are not being used, which can also be a security risk).

Good Luck, and Happy New Year!

Scott