[c-nsp] Ethernet debugging...MTU?

[Frank Bulk]

This mturoute program may be helpful:
http://www.elifulkerson.com/projects/mturoute.php

Regards,

Frank

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Pekka Savola
Sent: Saturday, October 27, 2007 12:20 PM
To: Andy Dills
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Ethernet debugging...MTU?

On Fri, 26 Oct 2007, Andy Dills wrote:
> Cliff notes:
>
> Is the inability to get ping replies with datagrams of larger than 9216
> bytes across a 100mbps ethernet circuit an indication that the far end is
> setup with an MTU consistent with jumbo frames?
>
> What to do if the far end swears it's set for 1500?

(Note that 100mbit/s FE interfaces may also support jumbo frames so
the fact that it's FE interfaces isn't a guarantee that it won't be
using 9216 MTU.)

When you ping with 9216 bytes, most likely your router fragments the
ping packets to multiple datagrams (maximum at 1500 bytes; verify
this).  When the target responds, how are the response datagram(s)
fragmented?  You should be able to figure out the path MTU by checking
the largest size of the fragments received.

You may need to ping from a unix host in order to more easily diagnose
this issue (e.g., run tcpdump to see the fragment offsets).

Based on that you should be able to figure out what IP mtu the remote
end is using.

If it's set to 1500 but ping (with > 9216) still fails the reason
might have more to do with the fragmentation capabilities/policies of
fragmenting devices (routers or the hosts at each end).

Working until 9216 and then stop sounds as if some device(s) is using
jumbo frames and a) you're pinging with DF bit set or b) some device
refuses to do fragmentation properly (or even at all) when the total
size of the full packet is too big.

--
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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