[c-nsp] MTU Question on T1

[Bill Wichers]

> 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