[c-nsp] Input queue drops due to backplane congestion

[mack]

We have a 6509 with sup720 3bxl running SXF11.
We have a number of 6704-10GE line cards and
A number of 6748-GE-TX line cards all with PFC3BXLs.

When one (only one) of the 6704 fabric channels peaks at about
80% utilization coming from the fabric we start to
See packet loss on the GigE ports.  Even if the traffic
Is confined to one GigE blade.

We have run a large number of troubleshooting exercises
Including replacing Sup units and line cards.  The problem
Is definitely fabric congestion.  Our current solution is
Simply to add another line card and move one of the lines
On that fabric channel.

This is somewhat different from the expected behavior
Which would be drops occurring on the fabric channel counters.

Has anyone else run into this problem?
Is this unique to SXF11? (we are planning an upgrade)
Would a 7609 chassis perform better with the same line cards?
Would changing the 'fabric buffer-reserve' to something other
Than the default queueing setting help?

I have only seen one other case of something similar on the list
Back in January.
Has anyone else run into this barrier?

--
LR Mack McBride
Network Administrator
Alpha Red, Inc.