[c-nsp] Cisco Processing Regarding ICMP

[Alexandre Snarskii]

On Sun, May 11, 2008 at 05:58:01PM +0200, Gert Doering wrote:
> On Sat, May 10, 2008 at 03:39:23PM -0500, alaerte.vidali at nsn.com wrote:
> > Because internal network design requirements, it is necessary decrease
> > internal MTU to slight lower than 1500 bytes, 
> 
> Ugh.
> 
> This is *really* unusual.  Many networks increase their MTU to well
> above 1500, so that even tunneled connections still are able to carry
> full-MTU packets - but running a network below 1500 sounds like a 
> Really Bad Plan to me.  

It's not so *really* unusual. Some parts of access layer in our
network is PPPoE over some 'really cheap' switches, which have no 
option to support MTU of 1504 (1500 + PPPoE overhead). 

> Expect fun with all the sites out there that have Issues with PMTUD.  Lots.

'ip tcp adjust-mss' helps. Really helps. I never heard about MTU issue
for years we running PPPoE...