Changing Sup720 from dcef to bus mode

Unanswered Question
Aug 8th, 2007


if i use the following command

" fabric switching-mode allow bus-mode "

1, will it reboot the chassis and sup720?

2, is this the right command to change the switching mode from dcef to bus mode.

I have this problem too.
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wochanda Wed, 08/08/2007 - 06:04

1. It will not reboot the chassis/sup720, but it will require you to reboot the system to take affect

2. It wont force DCEF linecards to operate in bus-mode. This command was designed as a workaround for bug CSCee10005, where we had packet loss issues with distributed etherchannels and service modules. This command will only force service modules (FWSM, IDSM, etc) into bus-mode. It will not affect the functionality of LAN linecards.

Jon Marshall Wed, 08/08/2007 - 06:22


We had problems with the FWSM and this bug. From memory i don't believe you have to reboot the system but once you enter the command the modules that get affected eg FWSM, IDSM etc. reboot themselves.

As you say the linecards are not affected and will not reboot.


wochanda Wed, 08/08/2007 - 06:46

Good catch Jon, you're right about this. I guess its been a while since I played around with that command.

malkova Sat, 11/10/2007 - 21:21

Same packet loss issue/bug on our fwsm as well.

As per TAC case,

we did a "fabric switching-mode force bus" which didn't reset the fwsm module(in our case, 6500-e), but "sho fabric" shows bus mode for sup720 and fwsm already.

we also recycle the fwsm, just incase to see if the packet loss issue goes away, but it didn't.

Still working with the support to see what else could do to resolve packet loss on fwsm.

Jon Marshall Sun, 11/11/2007 - 02:11


There is now an IOS for the 6500 that means you do not have to use the "fabric switching-mode force bus" command.

From Field notice doc



The recommended solution for native IOS systems is to upgrade to release 12.2(18)SXF7 or later which will remove the requirement to run the traffic in bus mode.


Full link to field notice

I haven't upgraded to this IOS so i can't recommend without you testing it fully.




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