[CALUG] Gigabit Ethernet

Ray Lischner linux at tempest-sw.com
Tue Oct 18 15:37:09 CDT 2005

On Tuesday 18 October 2005 07:00 am, you wrote:
> Wouldn't PMTU discovery allow the sending host to retransmit at 1500
>   for anything not destined for the local LAN?

I've been doing some reading, and I think you are right. Because my 
router would respond to an MTU discovery request with 1492
(because my ISP uses PPPoE), even my Jumbo-enabled adapters would then 
do the right thing.

My guess is that the sites who were having problems have Jumbo-enabled 
routers that are connecting to ICMP-blocking servers. Their MTU 
discovery requests go unanswered, so they merrily send along Jumbo 
packets, which are summarily dropped once they leave the Jumbo-safe 
environs of the university or whatever. At least that's my guess why so 
many people report that they need Jumbo end-to-end.

Thanks for the help.
Ray Lischner, Author of C++ in a Nutshell

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