[c-nsp] MTU Question on T1

[Richard J. Sears]

Hi Bill - 

I am sure that they are messing it up, but they have not identified that
error as of yet, even with me explaining it to them.

All they say if to do a 40000 byte packet ping from a windows box to
another windows box across the T1 and until that comes back below 50ms,
don't bother them!!



On Mon, 24 Jan 2005 19:47:08 -0500 (EST)
"Bill Wichers" <billw at waveform.net> wrote:

> > Right now the equipment is not working as advertised. According to the
> > vendor, this is because a ping of 40000 bytes is returning a rtt of
> > 400ms across the dual T1s.
> 
> Are you sure the vendor doesn't mean kilo-BIT? As others have said, a T1
> can only move about 190 or so kB/sec, so even with "ideal" performance, an
> mpp bundle with two T1s can only move 40 kB to one end and back in about
> 210 ms (on a very short link too). IMHO, intracity T1 links are usually in
> the 4-8 ms range for RTT (with an IP ping), and thats with a 7576 w/ RSP4
> on one end (our aggregation router), and any little cisco on the CPE end.
> Intercity links will have longer RTTs due to the increased distances
> involved. These are all physical limits and have nothing to do with MTU or
> anything you may have configured on either end of your link.
> 
> BTW, with a 40 kb (kilo-BIT) ping, you'll see more around 21-30 ms RTT.
> That's about a 5 kB packet. I'm thinking your vendor may be messing up the
> old convention of capital 'B' meaning 'bytes', and lower-case 'b' meaning
> 'bits'.
> 
> > Their entire claim is that because they cannot ping the other router
> > with a 40000 byte packet with less than 50ms latency, that is the entire
> > problem and until we fix that issue - don't call them back.
> >
> > So - my brain begins to think and I go to my 7500s, ping any number of
> > customers with T1s with no traffic on them with Cisco's max packet size of
> > 18024 - I see around 225ms.
> 
> 
> Your brain thunk correctly :-) Call back your vendor and play the part of
> the BOFH and inform them that they do not know what they are talking about
> network-wise, and that they need to fix their software.
> 
>      -Bill
> 
> *****************************
> Waveform Technology
> UNIX Systems Administrator
> 


******************************************
Richard J. Sears
Vice President         
American Internet Services                          
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