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GSR traffic shaping droppig packets

Scenario: Customer has a 1.5 Frame by ATM circuit connected to a 7500 ATM edge router, with VBR-NRT (remember this) configured on the ATM side (with policing enabled). This current configuration works perfectly with no packet loss. Of note is that this customer is one of at least over 100 pvc's on OC3 ATM interface on the 7500.

We then move the customer (and all the other pvc's) over to a Cisco 12000 Series Eight-Port OC-3c/STM-1c ATM Line Card. So all configured parameters for this customer are the same, it's just the ATM connection is on a completely different router. Now the customer is experiencing packet loss BUT not from the directly connected ATM 12000 GSR edge router, but instead from one hop away (a backbone router upstream).

An extended ping from the Backbone router to ANY interface on the GSR is good with no packet loss. However an extended ping sourced from the Backbone router to the customer's router (anything attempting to traverse the ATM cloud) will exhibit packet loss).

ping from GRS to CSR is good

CSR<<<<<<<<<<<GSR BB

ping from BB to GSR is good

CSR GSR<<<<<<<<<<<BB

ping from BB to CSR drops packets


Keep in mind this particular phenomenon does not occur when the customers pvc is located on a 7500 series router, it only appears on the 12000 GSRs.

The "fix" for this issue is to remove traffic shaping from the customer's pvc (thus it defaults from vbr-nrt to ubr). In which case there is no longer any packet loss when pinging across the ATM network from one hop away (i.e. pinging from the backbone router to the customer router). Or to move the pvc back to the 7500 series router.

Things to keep in mind:

1. Customer is not required to make any changes on their router as the cut over is logical on the remote ATM edge router side. Therefore the local loop remains unchanged.

2. Customer pvc's on both the 7500 and 12000 edge routers are configured the same as well as all traffic shaping parameters (in this case configured for vbr-nrt)

3. So the only change to this customers service is a move from a 7500 series router to a 12000 GSR line card (a change that moved this customer AND over 100 other pvc's).

Interface and memory buffers on the line card have already been checked and everything is within normal parameters.

I do remember an issue with the 4 port ATM cards being unable to handle only 4 instances of vbr-nrt, but according to Cisco the 8 port ATM line card should not have this limitation. My theory (although completely unfounded) is that perhaps the line card cannot handle over 100 pvc's all running vbr-nrt at the same time.

Any thoughts or information on this odd issue would be greatly appreciated.

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