How to configure RSTP on Catalyst 3650 & 2950

[algonzalez83]

Hi,

I need help configuring a network with Rapid Spanning Tree Protocol. There's a combination of Cisco 3650s, 2950 and 2960 switches. It is a flat Layer 2 network with a single VLAN. Does anyone have suggestions on the CLI configuration?

Any help would be much appreciated; thanks.

[Peter Paluch]

Hi Alvaro,

Running the RSTP on a Cisco switch is basically a matter of a single global configuration command:

spanning-tree mode rapid-pvst

In RSTP, it is very important to configure the edge ports - the ports towards end stations, notebooks, PCs, servers, network printers, etc. - as PortFast ports to make sure they will not be affected by topology change mechanisms of RSTP and will continue be forwarding even during network reconvergence. You can either do it on a per-port basis using the command spanning-tree portfast or (my personal recommendation) in the global configuration using the command spanning-tree portfast default

Also make sure that all your links are recognized as point-to-point (P2p) in the show spanning-tree output. RSTP is capable of rapid convergence only on point-to-point link types. The other link type, Shared (Shr), would resort to slow convergence. If there is a port identified as Shared port please verify its duplex setting, as the Cisco deduced the link type from the duplex mode of the interface. P2p ports should be runnning in full duplex (as that is typical for point-to-point links between switches with no hubs inbetween).

If you are migrating a production network, you should allocate a maintenance window for this, as transitioning to the RSTP will cause intermittend interruptions in the connectivity. You may be interested in reading the following two documents regarding migration to RSTP and the RTSP functional overview:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b0670.shtml

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

Aftter migration, verify again using the show spanning-tree that none of the peers is recognized in the output as still running STP - such an occurence would displayed in the line with the particular port to that neighbor and a comment P2p Peer(STP). If this indication remains even though you are sure that both your and the neighboring switch are configured for RSTP, use the clear spanning-tree detected-protocols command to reset the backward compatibility detection and let the switches rediscover themselves.

I am not aware of any other outstanding caveats - all your switches should run RSTP just fine.

Best regards,

Peter

[algonzalez83]

Thanks Peter, I'll try it out once the proposed change is approved by our client.

[petercrowe]

I do believe Mr. Gonzalez needs to mark this topic as "Answered" to give Peter due credit for his excellently concise, and clear answer. Nicely done Peter.