[ale] SSH Cisco Networking Issue

[Michael H. Warfield]

On Tue, 2010-09-28 at 05:30 -0400, Paul Cartwright wrote: 
> On Mon September 27 2010, Michael H. Warfield wrote:
> > You MIGHT try "ping -M do -s 1500 host" and see if it breaks.  The "-M
> > do" says do prohibit fragmentation (don't ask - I don't know why it's
> > that way) and the -s 1500 sets the packet size.  Back it down till it
> > works.  If it does, you have your smoking gun.  Still, I'm not sure I
> > can guarantee the test.

> so, I am an atnex.net customer, and I tried that with this line:
> ping -M do -s 1460 atnex.net
> PING atnex.net (208.65.89.2) 1460(1488) bytes of data.
> 1468 bytes from www.atnex.net (208.65.89.2): icmp_seq=1 ttl=124 time=51.4 ms
> 1468 bytes from autodiscover.atnex.net (208.65.89.2): icmp_seq=2 ttl=124 
> time=50.5 ms
> <SNIP>

> --- atnex.net ping statistics ---
> 7 packets transmitted, 7 received, 0% packet loss, time 6022ms
> rtt min/avg/max/mdev = 50.198/50.853/51.470/0.446 ms

> with anything higher I got this:
> From paulandcilla.homelinux.org (192.168.10.2) icmp_seq=2 Frag needed and DF 
> set (mtu = 1492)
> ^Cndcilla.homelinux.org (192.168.10.2) icmp_seq=2 Frag needed and DF set (mtu 
> = 1492)

> --- atnex.net ping statistics ---
> 1 packets transmitted, 0 received, +3908 errors, 100% packet loss, time 2459ms

> so should I set my router to 1460? I had always used 1492, but I really can't 
> remember why!

1) That -s is the payload size.  Don't forget the size of the smtp
header in there.  I really shouldn't have written -s 1500 but I was
typing fast and wasn't thinking.

2) You are getting "Frag needed and DF set" so PMTU discovery should
work properly and you don't need to artificially reduce your MTU
anyways.

The time you would need to fine tune the MTU is if you were getting
timeouts.  Both the cases you described above, everything is working
fine.  Leave it alone.

Regards,
Mike
-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
   /\/\|=mhw=|\/\/          | (678) 463-0932 |  http://www.wittsend.com/mhw/
   NIC whois: MHW9          | An optimist believes we live in the best of all
 PGP Key: 0x674627FF        | possible worlds.  A pessimist is sure of it!
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