[c-nsp] Sup720 - FIB full, software switching

Chris Welti chris.welti at switch.ch
Mon Feb 3 09:25:44 EST 2014

Simple answer: No.
One of the major design errors of the FIB in the Sup720.
Unfortunately, once the FIB is full, the only way to get it back to normal is to restart the whole box.

"CFIB-SP-STBY-7-CFIB_EXCEPTION : FIB TCAM exception, Some entries will be software switched
%CFIB-SP-7-CFIB_EXCEPTION : FIB TCAM exception, Some entries will be software switched
%CFIB-SP-STBY-7-CFIB_EXCEPTION : FIB TCAM exception, Some entries will be software switched

This error message is received when the amount of available space in the TCAM is exceeded. This results in high CPU. This is a FIB TCAM limitation. Once TCAM is full, a flag will be set and FIB TCAM exception is received. This stops from adding new routes to the TCAM. Therefore, everything will be software switched. The removal of routes does not help resume hardware switching. Once the TCAM enters the exception state, the system must be reloaded to get out of that state. You can view if you have hit a FIB TCAM exception with the following command:

6500-2#sh mls cef exception status

Current IPv4 FIB exception state = TRUE

Current IPv6 FIB exception state = FALSE

Current MPLS FIB exception state = FALSE

When the exception state is TRUE, the FIB TCAM has hit an exception.

The maximum routes that can be installed in TCAM is increased by the mls cef maximum-routes command.

This issue is common when trying to route a full BGP table on PFC-3A or a PFC-3B.

**Note a failover of the supervisors in dual supervisor system will not recover this exception, even through the “show mls cef exception status” will no longer indicate a FIB exception.  A full reload of the switch is required. "


Am 03/02/14 15:09, schrieb "Rolf Hanßen":
> Hi,
> today I saw 2x Sup720-3B (default 192K IPv4 routes) that received a full
> table.
> After FIB was filled IOS gave a warning that it now may forward in
> software (and resetted all BGP sessions because of memory issues). I don't
> have the exact messages.
> The real problem occured after that. I shut the full table BGP session and
> cleared the others, the system now had a few routes only again.
> But it started to drop packets, I saw no pattern, it looked nearly random.
> I needed to reboot both boxes to resolve that issue.
> IOS was s72033-advipservicesk9_wan-mz.122-33.SXJ.bin
> Is there a way to avoid those issues by let it just ignoring routes not
> matching into the FIB?
> Is there a command to reset the routing mode/routes back to CEF without
> reloading the box?
> kind regards
> Rolf
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