[c-nsp] ATM Issues

[Mark Rogaski]

An entity claiming to be clayton at mnsi.net (clayton at mnsi.net) wrote:
: 
: Both routers are acting as bridges (ATM Subinterface sits in the same bridge group as a
: GigE Sub Interface).
: 
: For some strange reason, I get occasional packet loss, particularly during periods of
: higher (30 meg or so) activity.  The PVC is set in the carrier's network for 100 Meg
: UBR service.


The first thing to ask is how many links the are in the path ... are you
sure the packet loss is on the ATM link?


: 
: I understand that the carrier may drop cells due to the class of service we're using,
: but the funny part is that I can't see any AAL5 errors on the PVC.

The carrier may drop cells regardless of what class of service is used, the
class of service just specifies how to determine what is considered
compliant or non-compliant traffic.  


: 
: I would expect that if the carrier is dropping cells, we'd see the AAL5 error counters
: increment.
: 

Not necessarily, if the carrier is doing early packet discard (EPD) all
cells in the AAL5 PDU will be discarded and no AAL5 errors will occur.  If
the carrier is doing partial packet discard (PPD or tail discard),
non-compiance can be tagged for any cell in the frame ... all cells except
the last one will be dropped.  The last one is left to prevent the next
frame from looking like an oversized frame.

EPD makes sense for a UBR service, when the ingress queues build up to a
certain threshold entire frames are discarded and oversubscribed bandwidth
isn't wasted transporting partial frames that the customer will discard
anyway.

Mark

-- 
[]                           |
[] Mark Rogaski              |         Computers save time like kudzu
[] wendigo at pobox.com         |         prevents soil erosion.
[] mrogaski at cpan.org         |                      -- Al Castanoli
[]                           |
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