Adding new switch to LAN and Spanning-Tree

[tobin_jim]

I have a LAN that has uses two 4500s and multiple lower-end switches. The 4500s are doing inter-vlan routing. Some of the lower-end switches use classic spanning-tree whilst the rest use RSTP. If I am adding a new switch to the network with trunk connections to both the 4500s, and the new switch is using RSTP, what effect will this have on the current users on the LAN. I will be stretching all my vlans across the trunks.

I'd imagine that adding the switch and causing new ports to go into a fowarding state would cause a STP topology change for the whole LAN.

Will this cause users to lose sight of the network? What about users on the switches running classic STP?

[Mathias Garcia · Accepted solution]

If you have configured the two 4500s to be root and backup root. (lowered the default priority)

Then adding a new switch connected to them should not cause STP to recalculate.

They will simply bring up the new ports. I do think that a TCN might go out and flush mac tables but that shuld be about it.

You might want to check that the new switch does indeed have a higher priority value than the 4500s before you patch it in.

RSTP 802.1w is backwards compatible with 802.1d (classic spanning tree) and will simply put ports towards such switches in a compatibility mode.

HTH

[tobin_jim]

Thanks for the response. The 4500s are primary and secondary roots for the vlans so that's good news.