SPT Convergence Time

[lejack99]

Hi,

We have 50 Catalyst 6509 and some other smaller switches connected to dual core 6509. Currently all the swicthes are in Transparent mode and I am thinking to migrate to Server/Client mode due to the configuration inconsistency and the benefit of VTP Pruning. All the STP values are in default of 2s for hello, 15 for forward delay, 20s Maxage.

My question is, in such a swicth network, what is the impact of changing to server/client mode? How much time is needed for the convergence of STP if changes are applied on the network. Also, I am in a very time-sensitive environment.

Thanks!

[kkalaycioglu]

I don't know about relation between VTP and STP but a VTP server announces on VLAN changes as soon as they're made. With timers you mentioned it can take 50 seconds for a port transition blocked->forwarding. Using some enhancements like portfast and uplinkfast you can shorten this time for some configurations.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094641.shtml

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008007f989.html

And new version STP : RSTP (Rapid Spanning Tree Protocol - 802.1w) is really faster converging STP. Check this:

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

Hope this helps.

Regards.

[milan.kulik]

Hi,

there is one relation between VTP and STP: trunk negotiation. If trunk negotiation is enabled on your switches and you are changing VTP domain name (or changing between transparent and server/client which is also a VTP domain name change in fact) your trunks might flap.

But I understand all switches are in transparent mode at the moment. So trunk negotiation should be off on all trunks.

Generally: STP convergence time depends mainly on your network diameter.

See http://www.cisco.com/warp/customer/473/122.pdf for details.

Regards,

Milan

[s.srivas]

Insist on doing the changes after hours. Read all about it, do a piolot migration (on the live network!), then do the full migration.

Do not forget to share your exprience with us.