Sunday, March 7, 2010

Dell Powerconnect 6224 slow vlan routing problem

So I noticed that intra-vlan network performance had decreased significantly recently but the effects were sporadic. Wireshark traces showed a lot of traffic bleeding over from multiple vlans and file copy performance between vlans was a staggeringly low. I ran the a cable qualification device on all the links and tested the speed between each switch. Everything was fine as long as I didn't do an inter-vlan activity. So I knew then that the problem was in the 6224 which I use for my layer 3 switching between vlans. While trolling through forums, I noticed someone had recommended STP as a place to start in troubleshooting. I set my 'root' switches priority 8192 (lower is higher in priority) and all the issues disappeared within a few seconds.

As far as I can tell the Multiple STP operation mode was running into problems when a load was applied. Since all the other switches are by default set to 32768 by default, now I shouldn't have this problem again.


Anonymous said...

Thank you so much. I have spent a week on this!

James said...

Chances are this was not STP going haywire but going through many topology transitions due to some sort of network instability or overloaded CPUs. The big question is, how "wide" is your network. If there are more than 7 switches in any given daisy-chain it's quite likely that not all switches will have agreed upon a single topology within the timeout periods and you can have multiple toplogies active at once and loops are formed. Then when any port transitions a topology change notification is sent and the whole process happens again. I've seen topology changes on the order of 10 per minute completely destroy network functions.

Properly selecting your root bridge (by giving it lowest prioirty #) and a secondary root (by giving it a slightly higher #) is critical in any network with more than a few switches. This allows all switches agree easily since there is only one best topology (and no elections are needed to see who has the lowest MAC address) and minimizes topology changes.

Arathon7 said...

Excelente, fue de gran ayuda.

Anonymous said...

This saved after 2 weeks of headache. THANK YOU!