Travis,
Comments in line...
> Hi List.
>
> We're testing some backbone new connections, ATM OC12 on GSR to Lucent
> GX550, running vbr-nrt. I've noticed extremely variable
> latency going from
> interface to interface across the ATM ... something on the order of:
>
> Success rate is 100 percent (100/100), round-trip min/avg/max
> = 36/46/56 ms
What are your SCR and PCR values? What is the propagation delay on the
link?
> When we try the same test on a UBR interface, default
> 100-byte packets, we
> see the much more normal:
>
> Success rate is 100 percent (100/100), round-trip min/avg/max
> = 36/36/40
> ms
>
> This behavior is pretty consistent across multiple links.
> Has anyone seen
> any problems with vbr-nrt on Cisco? I'm trying to figure out
> if it is a
> Cisco problem or something with the GX.
We've seen some things before, but they are often a problem with the carrier
switch. Lucents CBX line often is the culprit. In this case, however,
there should be no buffering due to shaping delay because of a full bucket
(your delay is high enough to drain the bucket for a one-hundred byte
packet.)
> As I understand vbr, the traffic isn't buffered at the router
> (although it
> apparently shapes, else what is the purpose of setting pcr
> and scr?) but
> is heavily buffered on the ATM switch itself. I'm wondering
> if this is
> normal behavior for a vbr setup on the switch, or on the router, or if
> someone hosed a software implementation.
Buffering happens on both devices, but for different reasons. On the
router, the tx_ring empties based on scheduler information fed from the SAR
driver. If the tx_ring fills, then packets sit in the SRAM buffers until a
free tx_ring slot opens. This is shaping.
On a switch (assuming no shaping is being performed), buffering happens per
service class with different scheduling priorities per class. The buffering
is high on VBRnrt links to accomodate bursts and cell-clumping resulting
from upstream congestion. This is controlled by the Burst Tolerance and the
Cell Delay Variation Tolerance values, respectively.
In order to understand this fully, it is best that you get a copy of
"Quality of Service in ATM Networks" by Giroux and Ganti. See
http://www.amazon.com/exec/obidos/ASIN/0130953873/o/qid=982127123/sr=8-1/ref
=aps_sr_b_1_1/105-9507683-4846311
>
> Any tips are greatly appreciated.
I really don't see a major issue here, although the delay jitter is slightly
anomalous. You may wish to open a case with the TAC and have them look into
a possible software defect in the driver code. Of course, Siva may have an
answer before you even get the case queued!
regards,
chris
>
> Thanks.
>
> -travis
>
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