[c-nsp] MTU Question on T1

[Richard J. Sears]

I have a interesting situation that I would like to run by everyone and
see if anyone has run into this before.


I have a vendor with two pieces of equipment. Here is the layout:



Vender Equipment -> FE on Router -> Dual Multilink T1s -> Router ->
FE on Vender Equipment



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.

Sh ints on all interfaces show no errors, no crcs, no drops, etc.

In our tests, there is nothing running on the link other than the vendor
equipment. They also suggested setting up LLQ (which we did) on the
routers and there was no change in the performance of their equipment.

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. 

Each T1 is set with an mtu of 1500 bytes, which means that every 40000
byte packet have to be broken up into almost 27 packets, get sent across
the T1, be put back together and then reverse that process in order to
get the RTT.

I do not see any place to increase the MTU on a Cisco T1 WIC card (if
even we would want to), nor do I see errors.

After looking at about 20 different T1s - I get the sense that I won't
see anything less than 300 to 400ms doing a 40000 byte ping test across
two Cisco 1720's.

I just want to see if I missed something or if my logic is off before
going back to the drawing board with these guys.

Any improvements in my logic or suggestions would be great.

Thanks !!




******************************************
Richard J. Sears
Vice President         
American Internet Services                          
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rsears at adnc.com
http://www.adnc.com
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