[c-nsp] Cisco Processing Regarding ICMP
Alexandre Snarskii
snar at paranoia.ru
Sun May 11 12:45:12 EDT 2008
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...
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