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Switching mode on 6500

I installed a WS-X6748-SFP in my 6509 with two sup 720 base and it changed the switchmode from flow-through to truncated which I believe is due to the fact that i had one non fabrice enabled blade WS-X6348-RJ-45 in it.

%SYS-3-FAB_CHNGMODE:System switching mode is being changed to truncated mode

in truncated switching mode my network performace severely degraded and we eventually had to remove these cards to restore services.

I want to know why it would degrade the performance, is it becasue it writes the first 64 bytes of frame over switch fabric and rest over the blade or something else?

What if I force to use bus mode? any caveates on that?

what is the best switching mode when i have fabric enable module but possibility that i can have one or two non fabric module as well.

Please advise.

Hall of Fame Super Blue

Re: Switching mode on 6500


I am a little confused about this. Flow-through is equivalent to bus mode. Once you added the WS-X6748-GE-TX the switch changed to truncated mode because of the presence of the fabric-enabled linecard.

Your network performance should not degrade because of the addition of the 6748. Truncated mode only effects the fabric enabled card ie. your other non-fabric enabled cards are still use the shared data bus to switch traffic.

We have 6500's at work running in truncated mode with a mixture of fabric and non-fabric enabled cards and there is no performance degradation.

How badly were services affected and what tests did you do to confirm it was the switching type that was causing the problem ?


Community Member

Re: Switching mode on 6500


Performance was degraded severely, we saw response time from 10ms to 100 ms, all monitors went down. I have read in the netpro that "In truncated mode switch will lower the maximum throughput of the system from 30Mpps to 15Mpps"and as soon as I removed the card, everything calmed down. My questions is, after all this happened my mgmt doesn't want to use truncated mode but interesting thing I have tried in the lab and forced switch to bus-only mode with the command "set system switchmode allow bus-only" but this didnt

work and switch still showed truncated mode even after the reboot. Any thought on that?


Hall of Fame Super Blue

Re: Switching mode on 6500


30Mpps is only available with compact mode. If you already had a non-fabric enabled module in your chassis before you inserted the 6748 then the best you could get would be 15Mpps which is what both flow-through and truncated use.

Could you find in NetPro where you read about 30Mpps to 15Mpps. As i say this will happen if your system is all fabric-enabled and running in compact mode and then you insert a non-fabric enabled card but this is not what you were doing.

So by inserting the 6748 you should not have changed throughput but the switching mode would change.

I have only really had to change the switching mode on IOS 6500 and on there you can set a threshold for how many fabric-enabled cards must be present before the switch can use truncated mode.


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