Migration from SPT to RSPT

Frederick Reimer · ‎06-15-2004

Is anyone aware of any documents on CCO of how to transition an enterprise network from regular legacy Spanning Tree Protocol to Rapid Spanning Tree Protocol? I found this document that describes RSTP, but it doesn't give any conversion advice:

http://www.cisco.com/en/US/partner/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

I have questions like, what happens when you set the STP mode to Rapid? If that is done on the root, will it cause a reconvergence/restart of all of the spanning trees (VLANs) configured? The mode is a per-port attribute, but what happens when you force protocol detection? Will that cause any outage on the network?

Thanks,

Fred Reimer

didyap · ‎06-21-2004

When a port initializes, the migration-delay timer starts and RSTP BPDUs are transmitted. While the migration-delay timer is active, the bridge processes all BPDUs received on that port. If the bridge receives an 802.1D BPDU after a port's migration-delay timer expires, the bridge assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs. So I guess there would not be issues while migrating since it would still work with the existing STP instance as you continue migrating one by one.

milan.kulik · ‎06-22-2004

But there is a little problem:

When you change the other switch port to RSTP, the first switch continues sending 802.1D BPDUs. Finally both ports are considered as 802.1D.

You have to use clear spanning-tree detected-protocols command on both sides to move the ports to RSTP mode.

See http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12120ea2/2950scg/swmstp.htm#wp1040474 for details.

The basic problem is there really is no migration document available on CCO.

And I'm afraid every STP mode change on any port can cause a line outage (up to 50 seconds) which can bring TCNs and STP tree changes to your production network. So definitely there is an impact to your network while migrating to RSTP from STP and I'd recommend to make this changes during some maintenance time.

Regards,

Milan