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New Member

Minimizing Network Convergence Time


I am setting up a campus switching solution for a client. The scenario is described briefly.

A collapsed core distribution model having 2 Nos Cat 4506 having and access layer switches 5 Nos Cat 3548XL and 3 Nos 2950G .

The Cat 3548 switches are uplinked to both Core 1 and Core 2 via Cat 5 Gigabit modules.

The Cat 2950 switches 1 ,2 & 3 are connected to each other via gigastack cables and switch 1 and 3 uplinked to the core via OFC and also via Cat 5 cables to Core 1 and 2 for failsafe as it is in a different floor.

There are 4 VLANS and no inter VLAN routing.

VLAN1 root primary is Core1 and secondary Core 2

VLAN 2,3 and 4 root primary is Core 2 and secondary Core 1

Backbone fast was not enabled in any switch as Cat 3548 is not supporting . Uplink fast was enabled with 1000 max rate interval in all access switches.

The convergence time was taking 30 secs initially without any tuning of the spanning tree network dia and hello timers. I tuned the SPT parameters and found the convergence time taking in 10 secs for the Cat 3548 XL series switches and less than 3 seconds for the Catalyst 2950G switches.

When I manually mapped the spanning tree per VLAN I,2,3 and 4 found that at any point of time the network diameter will not be greater than 4.

I am planning to tune the network dia to 4 and hello timer to 1 .so that the convergence time per VLAN primary root and the secondary be less than 10 secs.

Will that help to reduce the convergence to less than 10secs which is the requirement?



New Member

Re: Minimizing Network Convergence Time

The default spanning tree implementation on those platforms is PVST. I suggest MST for faster convergence. I've achieved almost and unmeasurable convergence time with MST. I've tested it with latencey sensative applications like ODBC for SQL and Oracle.

New Member

Re: Minimizing Network Convergence Time


Unfortunately the Cat 3548 does not support MST. So I had to implement PVST.. Cani tune the convergence less than <10 secs




Re: Minimizing Network Convergence Time


With Hello Time (HT) set to 1 second and Network Diameter (ND) set to 4, it is safe to tune your Forward Delay (FD) to 8 seconds and Max Age (MA) to 10 seconds. This will produce faster STP convergence times in the 16 to 26 second range (down from defaults of 30 to 50 seconds). Remember to make these changes on both of your 4506s.

Even if you could reconfigure the 2950Gs' connections back to the two core 4506s so you can get your ND down to 3, this would only let you safely adjust FD to 7 seconds and MA to 8 seconds, for STP convergence in a 14 to 22 second timeframe. This is still not in the <10 seconds window you're looking for.

The only way you're going to get STP convergence under 10 seconds by tuning FD and MA is if you get aggressive about it. Assuming HT=1 and ND=4, here's a sampling of the values you can try, along with the resulting convergence time:

FD=8, MA=10, STP convergence in 16 to 26 seconds (safe)

FD=7, MA=9, STP convergence in 14 to 23 seconds (aggressive)

FD=6, MA=8, STP convergence in 12 to 20 seconds (more aggressive)

FD=5, MA=7, STP convergence in 10 to 17 seconds (extremely aggressive)

FD=4, MA=6, STP convergence in 8 to 14 seconds (insanely aggressive)

By the way, FD=4 and MA=6 is as low as you can go; these are the minimum values.

While tuning the values into the "safe" range above is the least you can do, I would have thought that enabling UplinkFast on all your 3548s and 2950Gs would have helped each of them reconverge in 3 to 5 seconds in the event of a link failure on the Root Port. I'm kind of suprised the 3548s are taking that much longer.

In fact, the only time any of the switches in your network should experience a longer reconvergence time than 3 to 5 seconds is when one of the core 4506s goes down, forcing the other 4506 to take over as the Root Bridge for the VLANs that had been using the now-down 4506 as their primary Root Bridge.

Hope this helps.