[cisco-bba] High TCP packet loss over ATM

James Wakefield jamesw at deakin.edu.au
Sun Nov 5 17:03:20 EST 2006

Mark wrote:
> Hi ,
> i'm getting crazy about the problem below.
> We aggregate xDSL customers using PPP (PPPoA or PPPoE) over PA-A3 ATM 
> adapters.
> Now, some customers are experiencing very high tcp packet loss but when 
> we test ATM (with ATM OAM pings) we don't see any packet loss. It is 
> possible that it is a problem on the router? Only some customers are 
> experiencing the problem. We tried anything on the router (delete pcv, 
> change PPP type etc) but problem persist.
> I'm pretty sure that problems are on ATM provider, but on ATM side what 
> i need do check? ATM Buffers? ATM Errors? My ATM provider say that ATM 
> ping are enough to check that ATM network is working well but I'm sure 
> that problems are atm related. Maybe a IOS Bug?

G'day Mark,

Check the QoS parameters (Sustained Cell Rate, Peak Cell Rate, Maximum 
Burst Size) that your PVC is configured for against your ATM provider's 
SLA/technical spec.  If you're bursting faster/for longer than what your 
provider is configured for, it'll start dropping cells.  This will 
likely only be noticed with larger packets than your typical 64 byte 
ICMP echo, such as your typical ~1000 byte TCP packet.  Try pinging the 
IP interface on your ATM provider's end of the PVC with various packet 
sizes up to the MTU.


James Wakefield,
Unix Administrator, Information Technology Services Division
Deakin University, Geelong, Victoria 3217 Australia.

Phone: 03 5227 8690 International: +61 3 5227 8690
Fax:   03 5227 8866 International: +61 3 5227 8866
E-mail:   james.wakefield at deakin.edu.au
Website:  http://www.deakin.edu.au

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