RSTP tuning Convergence Time

Keith Burkhard · ‎06-03-2010

I have few Switches configured in STP mode RSTP with portfast default, portfast bpduguard default.

What can be done on SW1 for STP convergence time faster FOUR times the other switches?

I am thinking to change max-age to 5 second on SW1 as default on others is 20 sec. I think this is correct, any suggestions?

Should we change forward-delay timer also on SW1?

But following document Confuses me:

RSTP (IEEE 802.1w) natively includes most of the Cisco proprietary enhancements to the 802.1D spanning tree, such as BackboneFast, UplinkFast, and PortFast. RSTP can achieve much faster convergence in a properly configured network, sometimes in the order of a few hundred milliseconds. Classic 802.1D timers, such as forward delay and max_age, are only used as a backup and should not be necessary if point-to-point links and edge ports are properly identified and set by the administrator. Also, the timers should not be necessary if there is no interaction with legacy bridges.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#converge2

that means changing timers will not serve the purpose? any idea of what can be done for tuning convergence time on one switch orver the others?

Mohamed Sobair · ‎06-03-2010

Keith,

I havent seen in any documentation that you can tune Spanning tree Max age timer of the BPDU, However, if you want to achieve better convergence, I will recommend you th following:-

1- Set all edge ports to Spanning-tree (Portfast).

2- Set all half duplex ports to point to point edge port RSTP link type, Or change all half duplex ports to full duplex, that make them candidate for faster convergence.

3- Set Backbone fast and uplink fast features on the appropriate switches.

This will enhance your RSTP convergence,

HTH

Mohamed

Ganesh Hariharan · ‎06-03-2010

I have few Switches configured in STP mode RSTP with portfast default, portfast bpduguard default.

What can be done on SW1 for STP convergence time faster FOUR times the other switches?

I am thinking to change max-age to 5 second on SW1 as default on others is 20 sec. I think this is correct, any suggestions?

Should we change forward-delay timer also on SW1?

But following document Confuses me:

RSTP (IEEE 802.1w) natively includes most of the Cisco proprietary enhancements to the 802.1D spanning tree, such as BackboneFast, UplinkFast, and PortFast. RSTP can achieve much faster convergence in a properly configured network, sometimes in the order of a few hundred milliseconds. Classic 802.1D timers, such as forward delay and max_age, are only used as a backup and should not be necessary if point-to-point links and edge ports are properly identified and set by the administrator. Also, the timers should not be necessary if there is no interaction with legacy bridges.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#converge2

that means changing timers will not serve the purpose? any idea of what can be done for tuning convergence time on one switch orver the others?

Hi,

The nature of RSTP's fast convergence is the explicit handshake protocol. To keep convergence speed at maximum, you dont have to tune timers just make sure:

1) You have portfast enabled on all edge ports

2) All transit links are full-duplex point-to-point connections

3) Your "carrier-delay" is set to zero whether possible and if you are using any converters they can report link failure in timely fashion.

4) Your topology is as small and simple as possible. Avoid full-mesh or topologies with too much redundancy, as this may cause count-to-infinity problem upon reconvergence.

Notice that Max_Age timer does have impact on RSTP - it limits the topology size, as the number of hops cannot exceed the Max_Age. If it does, the root bridge information will expire in transit.

Hope to Help !!

Ganesh.H

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